list of abbreviations in newbies section-check it out.

It refers to cunnilingus or oral sex done to a woman.  It is a cute take off on having a dinner at the local YMCA or YWCA. (YMHA for you New Yorkers.)  So called because the two legs and the torso form a "Y". get it?

Another seldom used euphemism for cunnilingus is a "box" lunch.  When I first started out in construction, guys would tease me about whether I wanted to have a "box lunch" that day.  I had no idea what they were talking about and of course, they would all fall down laughing at the new kid.  When someone finally clued me in, I was able to fire back that I felt like "licking the can instead"  That shut them up for a while.