Its an acronym for Suck On My Hose.
it has the same use as Shut The Fuck Up ( as the person can't talk while sucking your hose ) or Fuck You, but the difference is that SOMH is more kind.
The inventor of this expression was me as i never saw someone sayin it and couldn't find it in the good internet searchers. I based it when i saw a South Park quotes, about how Cartman got a lot of semen, one guy just made him close his eyes and suck on a "hose", and voi la, there is the semen.
"Haha ur a big noob with big boobs! U hide in the wardrobe to watch ur sister dressing and play with your wee wee!"
"You just say a bunch of senseless shit, SOMH!"