Nightclub or urban slang. When a coyote is ensnared in a trap, it will gnaw its own limb off to escape.

After a long night of drinking, inebriated couples end up having sexual relations that might not have otherwise sober. When one or the other awakes in the morning only to realize they have made a dreadful mistake and regret terribly the choice they've made, (especially based on the ungodly physical appearance of their bedmate). Hence the term "coyote nightmare."