When someone (usually a male) hangs upside down while someone else does either one of the following given:
1) Gives you a blowjob
2) Gives you a handjob
3) Fists you. Usually associated with homosexuals.
When a female hangs upside down and you insert your dick in her vagina.
Guy 1," Get on the monkey bars and get into an underdog!"
Girl1 -"Orrr, You can get into an underdog and I can give you pleasure."
Guy 2 -"hey, can you get into an underdog?"
Guy 3 -"Sure."