Table and rules
Muck in poker
Definition
To discard your hand face down without showing it, either by folding or by giving up at showdown.
At showdown a player who knows they are beaten may muck instead of revealing their cards, which keeps information about their play secret. A winning hand must usually be shown to claim the pot.
Online, losing hands that are not shown are mucked automatically, and the hand history shows only what was revealed.
Example
Your opponent shows a pair of kings, and you muck your pair of tens without showing it.