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1. Coitus; sexual intercourse; the act of copulating.
2. A code word used for “coitus or sexual intercourse” in e-mails, which is less likely to get flagged by the company-installed software that checks for profanity.

From the word "butt," denoting one's backside, but also in many cultures, a word denoting sex + the "y" diminutive suffix; the children's word "poo," while denoting on the one hand, excrement, also connotes familiarity or intimacy between two people.
It was obvious to even the most casual observer that Kate Middleton and Prince William had long before their church-sanctioned marriage exchanged methane and enjoyed not a few instances of butty poo.

(From an e-mail) "No, sis, we saw a play at 7 p.m., had tea at ten, and of course retired to Buffy’s boudoir for some butty poo to round out the evening."
by T.T.T.--Gankinhisshit December 30, 2012
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