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1.
Adjective:

Very Very Smart. When the brain muscle is so jacked, you are unbeliveably Smart.

Modification of the Word Jacked, meaning very muscular or strong.
Person 1: Dude, I cannot Get this Question, its Redonk
Person 2: Bro, you just have to use the Binomial Expansion
Person 1: Holy Shit dude, your fucking Brain Jacked
by newom123 September 20, 2009
 
2.
when you have a repeating song or phrase that has taken control of your brain
i've been brainjacked by soldier boy
by nicholas barker March 05, 2008