A nasty yet entertaining trick played by men on women (or other men depending on how you are).
Committed in the closing moments of a blowjob, It is the act of grabbing the recievers head and jamming it down on your dick as you cum, thus making them choke/gag, re-routing your cum out through their nose running out of both nostrils. Number 11.
If you need an example, try it.
A new term for sex so parents and others, don't know what you're talking about.
"i heard that yesterday Shaunt'e went to Chris-stuff's house and they did number 11'
"EWWWW!!! i just walked in on my parents having number 11 !!!"