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basically it mean do you want to give me head (brain)
you would say it to someone if you wanted them to give you brain, or would use it to ask if you want someone else too.
jack: you on brain?
girl: yeeeaa
jack: sweet come over here now

Jack: you on brain with Danny?
girl: ooh NO
Jack: safee

girl: i am on brain with you.
Jack: waheyyy
by thegemmister January 02, 2010
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