look up any word, like kappa:
 
1.
When a male's subconscious went into the world of titties and football and ignored every VERY important thing the female said.
girl: "So Suzie said that she told Mary that Bob was... Are you listening to me?"

man: *no response*

girl: "Oh.My.God. You just totally manned out on me!"

man: "Huh?"
by NellieReno February 27, 2011
 
2.
the transformation of a young male's youthful appearance into an older male who looks around the age of 30 and older.
Jason, 28 years old, realized he had manned-out when his pic to a 23 year old on Craigslist was returned with the following message: "Can't you read? No one over 30!"
by Omar and Jason April 01, 2007
 
3.
when a young gay male becomes a grown man.
Jason just turned 29 but still felt 23. But he realized he had "manned out" when a younger East Village boy promptly told him, "You're cute, but aren't you like 30?"