A Buschwhacker is a term for NASCAR drivers who are regulars in the top-level Sprint Cup Series but who also compete in races in the lower-level Nationwide Series. The original coinage of the term Buschwacker comes from the fact the Nationwide Series was previously the Busch Series.
Because the Nationwide Series is essentially a minor league for the major-league Sprint Cup Series, this is a controversial practice. Critics say that Buschwhackers are racing against inferior competition and taking opportunities away from younger, less-experienced drivers, but many NASCAR experts contend that without Cup drivers in Busch and the large amount of fan interest they attract, the series would cease to exist. Because the Nationwide Series runs most of its events as undercard races for the Sprint Cup Series, there is almost no time conflict for Cup Series drivers who wish to compete. Cup drivers like to race in Nationwide races, which are usually run on Saturdays, as preparation for the Sprint Cup Series races, usually run on Sundays. Because Nationwide and Cup cars are only subtly different, the primary advantage of running both races is extra practice ("seat time") for the drivers. Some racing experts suggest that with Cup moving to the Car of Tomorrow the advantage of Cup drivers racing in Nationwide events will decrease greatly, this has not been shown to be the case. The last time a non-Buschwacker has won Nationwide Series points title was in 2005.
The presence of Buschwhackers is seen as problematic by some Nationwide Series regulars who complain about more talented Cup drivers taking the top prize money and thus leading to loss of sponsorship for Nationwide Series teams with no Cup affiliation. Also, in some cases, Cup regulars have better equipment thus taking away the regulars' chance to win and score much needed championship points. Other Cup drivers will run equipment independent of the Cup Series; in recent years, Tony Stewart, Joe Nemechek and Jamie McMurray have raced with Nationwide Series teams entirely separate from their Sprint Cup Series ones. While most Buschwhackers cherry-pick which Nationwide Series races to drive in, a few non-rookie Sprint Cup Series drivers, including Greg Biffle in 2004, Carl Edwards in 2005, and Biffle, Edwards, J. J. Yeley, Denny Hamlin, Reed Sorenson, Clint Bowyer, and Kevin Harvick in 2006 and 2007, attempt to run every Nationwide event while also competing in a full season of Sprint Cup, even though on some weekends the two series race at venues hundreds of miles apart. Some critics of Buschwacking cite the lack of Nationwide Series veterans left in the series, after long-time series veterans and champions (such as Steve Grissom, David Green, Randy Lajoie, Jason Keller, Casey Atwood, and Ashton Lewis) could not find a team to drive for as all the seats were taken by Cup drivers.
Still, the presence of Sprint Cup Series drivers in Nationwide Series races increases the exposure for the series and raises the attractiveness of the series for potential competitors as well as potential sponsors. It also increases the level of competition, and accelerates the development of non-Buschwhacker Nationwide Series drivers.
In 2007, only three non-NEXTEL Cup Series regulars won during the 35-race schedule: Aric Almirola, Stephen Leicht and Jason Leffler. (A fourth winner, David Reutimann made only 26 Cup starts out of 36 events: however, Reutimann missed 8 of the remaining 10 races merely because he failed to qualify, and was benched for a road course ringer in the other two.) Almirola's victory at the Milwaukee Mile on June 23 was controversial and unusual. Almirola was pulled from his car after leading 42 of the first 58 laps to make room for his team's regular driver, Cup regular Denny Hamlin. Hamlin missed the start of the race because he was delayed flying back east to Wisconsin from California, where he was driving a Cup event at Infineon Raceway the same weekend. Even though the car lost a lot of time during the driver change, Hamlin got back to the front to take the checkered flag after 250 laps. Almirola was officially credited with the win because he made the start.  
One of the Buschwhackers, Carl Edwards, clinched the 2007 Busch Series championship with two races to go. He ended the season 618 points over his fellow Buschwhacker David Reutimann. There were four non-Buschwhackers in the top 10 in drivers points: Jason Leffler, Bobby Hamilton, Jr., Stephen Leicht, and Marcos Ambrose.
Kasey Kahne has severely limited his Nationwide Schedule from 2008 to 2009. Kasey ran over 20 Nationwide races in 2008, and his first appearance of 2009 will be the Subway Jalepeno 250 at Daytona. He will run 3 races after that, the reason being that the Nationwide car doesn't resemble the Sprint Cup car at all, and the lack of funding and support, both mechanically and emotionally from Richard Petty, who has never supported Buschwhacking.
There have been some proposals made to restrict Cup regulars' participation in the Nationwide Series by not awarding championship points to such drivers. However, the points system in 2008 will be the same as in the past: i.e., all drivers will be treated the same regardless of what other series they race in.  Even though there is a widespread perception that drivers running both races hindered the development of new drivers, there has been a steady influx of successful new drivers into the Sprint Cup Series: e.g., in 2007 three of the Top 10 drivers had less than four years of Cup experience, eight of the ten had less than 10 years of Cup experience, and only one (Jeff Burton) was over 40 years of age. 
Cup drivers turn up in other series as well. Although it would be impractical for a Cup regular to run a full Craftsman Truck Series or Whelen Modified Series schedule, there are several who run trucks part-time. Kevin Harvick in particular is very active in the truck series: he owns a truck team which enters at least one vehicle in every race, and he frequently drives for his own team; and the most successful of these drivers is Kyle Busch, who has won seven races in the truck series. A term for a Sprint Cup driver who moonlights in the Truck Series is "Tailgater". Most of the leading Truck drivers have at least some Cup experience. Cup drivers also occasionally compete in regional racing series events.
The term originated in an argument Craig Witkowski had with another user "Tinadog" in the Usenet newsgroup rec.autos.sport.nascar in May 1997. Tinadog was against Dale Earnhardt and liked Mark Martin, and Witkowski the reverse. He was especially critical of Martin using his Winston Cup team and resources to beat up on the drivers in the lower Busch Grand National Series. The old western term of "bushwhacker" was morphed into "Busch Whacker". 
The term continued in use among the regulars on rec.autos.sport.nascar ("rasn" for short), referring now to any driver whose primary ride is in the Cup series and cherry-picks Busch races. The term was picked up by Fox Sports broadcaster Mike Joy, who also participated in the group. After Mike Joy used the term in his broadcasts, it was picked up by other members of the media and found its way into common use. Mike gave credit on the air on lap 64 of the Fox telecast of the Hershey's 300 Busch Series race at Daytona International Speedway in 2005.
Fox later discontinued the use of the term on its telecasts. This may have been done to save face among the critics of this practice, or perhaps because few of the leaders were non-Sprint Cup Series drivers anymore.
An alternative term was used by Larry McReynolds initially, as Nationwide is an insurance company, he dubbed the Sprint Cup invaders as "Claim Jumpers", punning off the term of an insurance claim and mining rights.