The muck is the pile of discarded cards kept by the dealer during the course of a hand of poker. It is kept in front of the dealer, facedown, and generally on the same side of the table as the hand where the dealer holds the undealt deck.
To muck a hand or a card is to toss it into the pile of discards. It means the same thing as discard.
Touching the muck Edit
Generally, poker rules hold that if a player's card touches any card in the muck in an unprotected fashion, the player's card and hand is declared dead and must be immediately mucked, as if the player had folded. This is why players who sit in a seat next to the dealer must be extremely careful in handling their cards to ensure that they never touch the muck. The rules about touching the muck are usually quite strict and very firmly enforced, to prevent the "accidental" mixing of cards from a discarded hand with other discards.