Definition of muck
"Muck" means two things in poker. When you call a bet on the river, for example, and your opponent shows you a hand that beats you, you have the option of showing your losing cards or mucking, which is throwing your cards away without showing them. Mucking is also known as discarding in poker. When you give up a hand and throw away your cards, you are said to be throwing away your cards in the muck.
When you have the losing hand, do you muck or not?
If you call a bet on the river and you see that your opponent has a better hand than you, should you muck (hide your cards) or not? The majority of the time in poker, we don't give our opponents free information. Showing our cards gives information to our opponent who can then analyze how we play our hands. But if you decide not to muck and show, make sure you have a good reason for doing so. If you are able to explain clearly why you are showing your cards to your opponent and it accomplishes something in your favor, then only then is not mucking a good idea.