Muck
Also known as: mucking, the muck, muck pile, to muck
To discard a hand face-down into the pile of dead cards; also the discard pile itself.
Muck has two senses. As a noun it's the pile of dead cards — folded hands and burn cards — at the dealer's side. As a verb it means to discard your hand, either by folding during the action or by surrendering an unwanted hand face-down at showdown rather than tabling it.
The critical rule lives at showdown. "Cards speak": a hand that is tabled face-up is read by the dealer for its true value, even if the player misreads it. But once cards touch the muck and become irretrievably mixed, the hand is dead — you cannot win the pot with it, even if it was the winner. This is why you should never release your cards until the pot is awarded, and why an opponent miscalling their hand to induce your muck is a classic angle.
Who shows first depends on the action: at an uncalled river the bettor can muck and take the pot without revealing; when there was a final bet and call, the last aggressor shows first, and players to their left may muck in turn if they can't beat the tabled hand. In an all-in, all hands are turned face-up immediately and the muck doesn't apply. Protect your cards with a chip or card-cap; a hand the dealer accidentally sweeps into the muck is typically dead, and the floorman rarely reverses it.