To muck is a synonym for fold. It also refers to the discard pile where folded hands are placed.

Using the term muck implies that a player did not show their cards and put them face-down in the discard pile. This applies additionally to hands which are discarded at showdown; the player is not compelled to show their cards at showdown and may choose to muck them instead of revealing. Technically one could muck a winning hand though for obvious reasons it is virtually never done.

Some online poker rooms allow other players to see mucked hands at showdown via the hand history.