Unofficial girl is a girl who is just a hook up with a guy.Nothing more than just a fuck buddy to him.Not in a relationship with the girl.
Like in Cassie and Lil Wayne's song...

And I'm tired I'm tired I'm tired of being your (your) unofficial girl unofficial girl und I'm tired I'm tired of trying(ooh but I'm still your)I'm still your unofficial girl.
by Jessica Rae{Slick} December 18, 2008
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