The practice (almost exlusively a male preserve) of exposing
intimate parts of your
anatomy to unsuspecting passers-by (usually female).
I suspect the origin of the term is that the stereotypical
flasher would wear a long macintosh
overcoat, buttoned-up, to disguise the fact that underneath he'd be wearing his trousers around his thighs, so that he can just "flash" his victim by quickly opening then reclosing the coat.
Once thought of as a fairly minor, if rather unpleasant sex crime, most psychologists and police officers believe that flashing is merely the first step on a road that can often lead to more serious and violent sexual behaviour. Not really a laughing matter.