To forget about someone or something, to let go, to get over it, to move on.
As in Mariah Carey's song:

"I gotta shake it off
Cause the loving ain’t the same
And you keep on playing games
Like you know I’m here to stay
I gotta shake it off
Just like the Calgon commercial
I really gotta get up outta here
And go somewhere
I gotta shake it off
Gotta make that move
Find somebody who
Appreciates all the love I give
Boy I gotta shake you off
Gotta do what’s best for me
Baby and that means I gotta
Shake it off"
by StefB86 May 18, 2006
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After a male urinates, he doesn't wipe like a female would. Instead he holds onto his genitalia and "shakes" it, so as to dry off.
He had to (shake it off) so he could put it in his pants.

*glancing around stupidly at your friends, miming the shaking off* "(SHAKE IT OFF)".

~(shake it off) can also be used as a dumb immature joke, as seen in example number 2~
by -S July 8, 2005
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The act of telling a girl to piss off or get lost.
OMG! Rachel is so annoying! I really need to shake her off!
by RobTheCrucible12 November 26, 2013
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The gentle shaking of one's penis after urination to insure that any remaining urine is expelled before pulling one's pants up.
Dave rememberd to shake off because he didn't want to stain his boxers.
by Sean Piece September 26, 2003
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Similar to spanking the monkey, waxing the carrot, choking the bishop, jacking the beanstock, yankee-ing the doodle,peting the one eyed wonder weasel, waxing the wood, caulking the cracks, jacking off, wanking.
"Claus! Stop Shaking off the anchovy, kids are in the room!!!"
by InfinityMan October 24, 2020
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It means going number 2 in the bathroom. In baseball, when a pitcher wants to throw a fastball, the catcher puts one finger down. If he wants to throw a curveball, he shakes his head to the fastball and the catcher puts 2 fingers down for a curveball. Also known as "calling for a curveball".
by CPAbel February 4, 2010
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