Yes, which brings it back to couple in the context of the joke being a nominative plural, thus making it "couple's"; and, hence, "a couple was...". However, when it is indeterminate, then it becomes plural, as in "a couple of cats were fighting..."
Yes? No?
Anyhow, is it "a pair of underpants were left lying on the floor", or "a pair of underpants was left lying on the floor"??
Yes? No?
Anyhow, is it "a pair of underpants were left lying on the floor", or "a pair of underpants was left lying on the floor"??