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.

the thing that you don’t have when you’re phones percentage is over 20

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...