I heard this term not long ago and I thought I should share it.
Basically a countrybagger is a rock or pop artist who decides to make a country song or album as a cheap ploy for cash or attention.
For example, "that single by Steven Tyler would be the worst example of countrybagging I have ever heard. "
or "Lionel Richie just released a country album. I would never expected him to be a countrybagger."
Obviously this is open to interpretation as to who qualifies or not. If you are a rock star with heavy folk influences, then you wouldn't be one. If you are a "hair metal" band from New Jersey that releases a country track, you just might be a countrybagger.