Definition of dying
In short, when you make a dying call, it's like saying to yourself: "I probably don't have the best hand, but it's mathematically winning to call, I have no choice". We can also make a dying call in a situation where we have (for example) the best hand 10% of the time, but given the size of the pot, we have no choice but to call. For example, on the river, a player bets, you have the ace flush and raise big. However, your opponent pushes all in. The board is even and this player is not bluffing much. Presumably, he has a full hand. You won't often have the best hand, but the pot is too big to give up, so you make a dying call.