The back and forth movement of an erectile penis moving back and forth "in and out" of a girl's vagina; it usually is a repeated movement when it comes to sex and pleasure for the lady.
Its nothing special, they're just doing the old in out, in out.
by Justin February 20, 2004
Slang; Nadsat for 'sex'
"I would have liked to have a bit of the in-out, in-out right there on the stage with her." - Alex (A Clockwork Orange)
by RandyL November 11, 2003