A particularly hideous member of the opposite sex.
When a wolf is caught in a trap it will commonly chew it's own leg/s off in order to escape.
Now imagine awaking the morning after a big night, to discover that the previous night's conquest is a hideous monster, and one's arm is stuck beneath her/him. One could quite easily think that it would be better to chew one's arm off and run away rather than risk waking said monster.
Friend: "How was last night?"
Trapee: "Awful, woke up with a wolf trap"