The idea of smacking someone with a heavy bag of sand is inherent in the use of the word in cards.
I've never heard it used with reference to poker, but it's common in pinochle. It means to pass [in the bidding] with a good hand. The objective is that if the dealer is well ahead in the scoring and likely to win in the next hand or two, you "stick" him with the bid. (In pinochle, the dealer bids last and if no one else bids, he must play the hand, no matter how bad; or, in some circles he can "throw in", i.e., be penalized the amount of the minimum bid without playing the hand and without the other players, including sandbaggers, being able to score meld.)
Hence, using the old rules where the dealer must play out the hand, a sandbagger not only has a good hand to assault the dealer with, but a lot of meld score as well, which is like coshing him.
