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."