I started the inappropriate hi-five. It was my thing. I designed and deployed it as a real world version of the much sought-after "dislike" button everyone wants in Facebook. Properly executed, it's a raised hand prefaced with the phrase, "inappropriate hi-five". Adjust intonation of phrase to suit the severity of the situation.
Buddy lost his job? Throw him an inappropriate hi-five. Buddy too drunk to drive home? Take his keys and throw him an inappropriate hi-five to cool his temper. Lousy performance in the bedroom? Spice it up with an inappropriate hi-five (results may vary).
If you've ever been offended by an inappropriate hi-five, whoops - my bad. If you find yourself in a situation where it's applicable, try it on for size...it is a thing, after all.