"SUM" is a shortened direct command with a meaning similar to "be quiet"', but which is commonly perceived as an angrier and more forceful demand to stop making noise or otherwise communicating. The phrase is probably a shortened form of "shut your mouth up", and its use is generally considered impolite.
Laura: I'll really hate rain, it scares the hell out of me. I mean were does it come from, can you drink it?
Sums is an acronym for Spontaneous Uncontrollable Masturbation Syndrome. Sums is caused by puberty, boredom, lack of action, high sex drive, hot people, ect. It is characterized by spontaneous and uncontrollable masturbation, often at inappropriate times such as at the park, when voting, or in line at the bank.
-"OMG look, he's doing it right there!"
--"He can't help it, he has sums"
-"he's my hero..."
A game where when someone says something stupid and you get to hit them on the head for it. After someone says the stupid statement, you immediately say "That's sum." while rubbing your fingers in the air like the sign for paying money.
Mick, "I drank 7 out of the last 5 days"
Al & Calvin, "That's sum, asshole (rubbing fingers in the air)."
then Al & Calvin each get to slap mick on the top of the head.