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1.
The act of emitting a sense of power towards a weaker person. To be overbearing or dominating towards an individual or group of people.
"Jason tried to flex nuts and got beat down."
by Cyko October 22, 2003
 
2.
Flex nuts. A term coined by the Geto Boys in their song "Damn it feels good to be a Gangsta"

The metaphorical act of flexing ones testicles to make them bigger than they are. however physically impossible this act is, it simply means to play tough when you feel inadequate.
Damn it feels good to be a gangsta
A real gangsta-ass playa plays his cards right
A real gangsta-ass playa never runs his fuckin mouth
Cuz real gangsta-ass playas don't start fights
And playas always gotta high cap
Showin' all his boys how he shot em
But real gangsta-ass playas don't flex nuts
Cuz real gangsta-ass playas know they got em.
by Gbark June 05, 2009
 
3.
To act tougher than you really are. When somebody tries to act tougher or dominating, but doesn't necessarily pull it off in a believable way.
When I made fun of Jason at the party in front of his girlfriend, he all tried to flex nuts and get up in my face, tellin me he was going to whoop my ass. I told him let's go outside, and he took his girl's hand and pussed out. He tried to flex nuts, then backed down.
by Jeffro April 10, 2007