Muck in gambling has multiple meanings. In poker, it most often refers to the pile of discarded cards into which players may throw their folded hands, and into which the dealer may place burned cards. It may also refer to the action of throwing a hand into the muck. When a player is folding his hand (face down) without saying anything, in fact the hand is not folded until it reaches the muck (it can be taken back and used if the dealer did not take the hand yet). The practice of mucking cards when discarding helps to ensure that no other player can reliably determine which cards were in the folded hand. In poker, the term may also refer to the action that a player who has not folded may take; he can have his hand "mucked" if another player attempts to discard but one or more cards end up in the live players hand. This is why many players will place a chip or other object on their cards: it helps to prevent errant cards from entering their hand. Sometimes they are referred to as card covers, card guards or card protector.
Mucking or hand mucking may also refer to a form of sleight of hand, and, if used in a card game, is cheating. A player conceals a card through sleight of hand, removing it from play so that it may later be inserted back into the game to the cheater's advantage. For example, in blackjack a cheating player might remove an ace from the table to use the next time he is dealt a ten to make a blackjack. 
- Lee Jones: Winning Low Limit Hold 'Em, 3rd Edition, page 263
- "oddsmonkey". Sunday, 7 May 2017