Definition of mourant
An call mourant is often an call that is done on the river, especially at the Hold'em limit. It can also be a 15 when a player is all in. An call mourant is an call that is made by thinking that it is probably mathematically a winner, even though we don't often have the best hand at call. We have the calls that we do was very happy with it, for example with the nuts on the river, we have the fold clear to do and we have the games halfway between the two. The call mourant is never easy.
Some more information about the call mourant
In short, when you make an call mourant, it's like saying: "I probably don't have the best hand, but it's mathematically a winner of call, I have no choice". You can also make an call mourant in a situation where you have (for example) the best hand about mourant% of the time, but given the size of the pot, you have no choice but to make call. For example, on the river, one player bets, you have the flush to the ace and raise big. On the other hand, your opponent pushes all in. The board is paired and that player doesn't bluff much. Presumably, he has a full hand. You won't often get the best hand, but the pot is too big to fold, so you make an call mourant.