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2.
Threat suggesting that the enemy should leave or be quite. The enemy may take the advice (see punked) or disregard the advice (see toe to toe or fight).
"I've got a bat with your name on it. Best for you to back the fuck up."
by Beedub January 31, 2004
 
1.
a hostile directive to retreat; usually to feign an attempt to prevent an altercation.
Nigga, You had best to back the fuck up offa me fo I beat yo bitch-ass!
by spence-doggy-ddog February 01, 2006
 
3.
What we told Japan to do but they refused so we nuked the shit out of them
President Truman: Japan better back the fuck up
Japan leader: fuck no
President Truman: ok then, boys prepare the atomic bomb
by applealex December 01, 2009