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When your girl trusts you enough to let you cum inside her for the 1st time. It's a play on the phrase "Keepin it 100" and comes the theme song from the Mickey Mouse Clubhouse where they say "Come inside, it's fun inside." M.M.C. in roman numerals is 2100
Josh: My girls keeps it 100 and never lies to me, she'll be a good wife one day.
Justin: Well my girl is keepin it 2100 and let's me bust a nut inside ever since I told her I love her on Valentine's day
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by Da B.E.A.S.T. February 07, 2017
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