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When you have to make a decision between someone you slept with or someone else.
Comes from the phrase "Blood is thicker than water"
Jill:I have to give a promotion between Joe or bob
Alex:didn't you go out with joe?
Jill:yeah but bob is equally qualified for the job...
Alex: I say cum is thicker than water.
by Shojonen November 07, 2010
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