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1.
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
 
2.
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
 
3.
Sex. From the movie "A Clockwork Orange"
Like a bit of inout I do.
by louis October 23, 2001