An older man, constantly whining and complaining (to anyone who will listen) about his long-lost youth. Constantly hangs around with the younger crowd and observes with resentment. This phase often precedes or coincides with midlife crisis.
Note: It is best to avoid a bitter bill in social situations, especially when alcohol is added to the mix.
"I avoid going to bars during happy hour, because at that time of day, it's usually just a bunch of bitter bills looking for young tail."