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This when some one says something really dirty, and you not expecting to hear something like that come out of there mouth.

Also can be used if you want to have clean ears, and the other person took it way to far.
Jane:Nick should put his d*ck in me and f*ck me deep and hard.
Sadie: Wow, don't cross the dirty line.

Jane: Sorry, I'll shut up.

Crossing the Dirty Line
by 임신했습니까 November 02, 2011