To discard or throw away your hand thereby folding and taking them out of play. Muck is especially used when discarding cards after an opponent reveals a better hand than your own. When players muck their cards they are returned to the dealer.
The term can also refer to the discard pile of cards no longer in play. The collection of face-down discarded cards gathered in the centre of the table near the dealer. The pile contains folded hands, burns and discards for drawing purposes.