Once you go blue, you can't undo.
The phenomenon in which a normal conversation turns blue, becoming so riddled with sexual innuendo that it's rendered nearly impossible to steer back in a non-pervy direction. Every innocent comment starts to become a dirty double entendre, until eventually the entire conversation turns into a complete smut-fest from which there is no return.
Girl: Ah, who doesn't love stripper zombies in Portland? That'd go down like Jesus on a velociraptor.
Guy: I have to admit, when I first read 'go down'... I had rather blasphemous thoughts.
~ten minutes later, during which this term is invented~
Girl: Hmm, this Urban Dictionary thing is quite fun. Something for the annals of history.
Guy: Two n's - one 'n' is bad.
Girl: Yeah, I always thought that 'annals' was an odd word.
It really could so very easily be another.
Guy: Terribly easily. So, how is the entry? ...Oh, Jesus!!!!
Girl: Once you go blue, you can't undo.
Guy: I have to admit, when I first read 'go down'... I had rather blasphemous thoughts.
~ten minutes later, during which this term is invented~
Girl: Hmm, this Urban Dictionary thing is quite fun. Something for the annals of history.
Guy: Two n's - one 'n' is bad.
Girl: Yeah, I always thought that 'annals' was an odd word.
It really could so very easily be another.
Guy: Terribly easily. So, how is the entry? ...Oh, Jesus!!!!
Girl: Once you go blue, you can't undo.