to be drunk, high, and to then have sex with 20 other people of the opposite sex as you at the same time. often after having potcheen. You would think it's a very memorable experience, but very often one blanks out and forgets that it ever happened.
Guy: that was a great 21 some last night.
Girl: what 21 some?
Guy: the one where you were drunk and high and sucking on my dick!