Muck
To fold without showing, or to toss losing cards at showdown.
Say you're on the button with Kc Qc and raise. The big blind calls. The flop comes Qh 7h 3d, giving you top pair with a king kicker. The turn is 9h, putting three hearts out. Villain checks, you bet, and get check-raised for close to the size of the pot. You hold no hearts, and the sizing reeks of a made flush. Top pair isn't strong enough to fight that, so you fold - and instead of tabling Kc Qc face up, you toss it into the muck, giving away nothing about how close the call was.
That silence is the point. A hand thrown into the muck reveals nothing - not your cards, not whether you were bluffing or folding the best hand by mistake. Good players muck by habit even heads-up with nobody watching, because every free look is information for the table. The word covers showdown too: if you're beaten, you don't have to turn a loser over card by card - you can tell the dealer to muck it, and it never sees daylight.
The common beginner mistake is treating the muck as reversible. Once cards touch the pile they're dead - the dealer won't dig them back out because you suddenly decide your two pair was good after all. The other classic error is flashing a hand as you muck it, to prove a point. Showing a big bluff or a slow-played set costs nothing in the pot, but it hands the table a read for free - exactly what mucking is meant to prevent.