Definition of muck
"Muck" means 2 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 "muck", i.e. discarding your cards without showing them. The "muck" is also the discard pile on the poker. When you give up a hand and discard your cards, you will be said to be discarding to the "muck".
When you have the losing hand, do you have muck or not?
If you call a bet on the river and see that your opponent has a better hand than you, should you mucker (hide your cards) or not? Most of the time on poker, we don't give free information to our opponents. Showing our cards gives information to our opponent who can then analyze how we play our hands. But if you decide not to mucker and show him, make sure you have a good reason to do so. If you are able to clearly explain why you are showing your cards to your opponent and that it accomplishes something in your favor, then just not showing mucker is a good idea.