A variant of ZOMG, itself derived from OMG, (an abbreviation of "Oh my God"). This is widely used whilst conversing on the internet, and is oft utilised in mild derision of users of the aformentioned phrases.
Unwitting victim: What?
A typo of 'ZOMG' (which is often sarcastically used towards people who say OMG alot, or are stating the obvious) When someone mis-types ZOMG as XOMG it often leaves them looking foolish, and as dumb as the person he/she was trying to make fun of. XOMG would usually be used by someone who is trying to look good infront of his internet buddies...
Person 1: OMG look that guy is so uber!!11!1
Person 2: (Sarcastically) LOLZ YE XOMG LOOK AT HIM! -_-
Person 3: (Even more sarcastically) Ye.. you sure made him look stupid with your typo...