What exactly does the string of words functioning as a preposition "all up in" (many many Google hits) mean in modern American slang usage?
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All up in is generally used to refer to someone being overly nosey, annoying or disapproving.
It can also be used more generally to show that something or someone is completely committed or present in a place or situation.
Green's Dictionary of Slang offers two possibilities, either "sexually involved with" or "interfering, involved with". The former is American campus slang and the latter American teen slang.