A bag or “holding a bag” is slang for owning some quantity of a specific cryptocurrency.
This can be a large amount or it could be small.
You could own a “large bag” of bitcoin (BTC) because you’re a Bitcoiner or a true believer of the blockchain.
Or as another example, upon the release of a brand new cryptocurrency, you buy a “small bag” of just a few coins of that currency to get exposure to something new and exciting, that has some real promise of getting popular.
Did you grab some of that GermanShepCoin, bro?
Oh definitely. After Elon tweeted about accepting it to buy a Tesla, I picked up a small bag. Moon baby!