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1.
The feeling on Valentine's Day when you do not have a Valentine, but your friends do.

Usually, the person is overcome by jealousy and depression.
A: "Hey bro, you got a Valentine?"
B: "Nah, I'm going to have a really bad case of Valentineitis when I think about what you and Sally are going to be doing.."
by Zakul February 12, 2008