1. That last great and final insult: given with the implication that the speaker has done something (usually sexual in nature)to or with the recipients mother-though not necessarily hostile in intent. 2. The phrase spoken in a last ditch effort to have the last word in an argument.
You're so stupid I bet your I.Q. is negative.
See also: yo' momma, your mother, tu madre.
NOTE: Although meant to be the last word in an exchange, I have, over the years, heard many a good reply to all of the above retorts. My favorite to date- I don't have a mother. Me and my dad share yours!