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When you date a girl, but you are ashamed to tell your friends about her because she is fat or ugly or something.
ME: "Hey, I really like you, will you date me?"
HER: "Of course!"
ME: "Great, but hey, uhm, could you be my Secret Girlfriend?

HER: Secret Girlfriend?
ME: At least til you lose some weight."
by Erik with a KKKKKKK September 19, 2009
The girl who is rely your gf and not anyone else no matter what and cant not be changed no matter what
dude1: im dating this girl kodi dude: no way bitch shes my secret girlfriend hands offf rofl copter
by acdc117 April 15, 2010