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1.
It is the joy of having a bonerific time in your pants. When your snake gets larger due to interactions of various human kinds, it can lead to the situation of 'Tight undies'.
An example of this:
Boy 1: Oh hey theres that chic I want to ask out, should I do it?
Boy 2: Yeah go for it dude
*Boy 1 walks over*
Boy 1: Hey babesickle, wanna go out this weekend?
Girl: Yeah sure, but what is that bulging in your pants, looks like your undies are getting a bit tighter
Boy 1: Well you see, old beatrice got a bit excited downstairs when I saw you, so yeah... Bloody tight undies
by Max Barnes September 02, 2013