Extremely crude yet oddly witty internet acronym, translates as 'Sodomizing My Nan Laughing'.
other variants include 'Domnl' (defecating...) and 'Pomnl' (pissing..). used in the corners of civilized society eg. universities, the houses of parliament.
Zacharias: Dude, did you hear about what happened to steve after he got drunk last night?
Herbert: Yes! SOMNL!