A barfly is someone who spends a great deal of time in bars, drinking and talking, almost living there. The picture is of a fly that hangs around food and will not leave. It is informal, mostly American, and usually a little unkind — not a proud job title, though some people use it with a shrug or a joke. You hear it in films more than in careful writing. A person who went to a bar once is not a barfly; the point is the habit, night after night.