Fly bar

(a series "what were you thinking")

context: a car is pulled over on a routine stop.
Sunddenly a man is splattered onto the car, and the narrator says:

The driver doesn't look drunk, and everything seems fine until... this *bar fly* gets splattered onto the car. Where did he come from?

My question? is *fly bar* a kind of slang? does he mean that the person had a lot to drink? (bar), or he means some object?

thank you very much!!

'Bar fly' may be used to refer to a person who spends too much time in barrooms.