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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
 
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
 
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