(Of a male) The act of not pulling out of a female's vagina when about to ejaculate; made worse by the non-usage of a condom.

This phrase is the opposite of the popular "Pull out before it feels good", and instead means to go past the point of no return (i.e. a creampie)

This may further result in pregnancy or a bag baby.
I was getting it in with my girl this weekend, when I decided to cross the rubicum. If she wasn't on the pill, I'd be a father for sure.

I accidentally crossed the rubicum with my friend's sister the other day. Thank goodness for Plan B!
by GerYard April 09, 2014

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