ZMG or ZOMG
Oh My God
A variation on OMG, it is a more enthusiastic and sarcastic way of saying OMG. Some say the Z doesn't stand for anything, but rather is added onto the O, making it pronounced as "ZOH MY GOD!! However, it actually comes from a gaming expression "ZERG OH MY GOD!" As in "they about to get pwnd by a mob."
It is online jargon, also known as text message shorthand, used primarily in texting, online chat, instant messaging, email, blogs, and newsgroup postings; these types of abbreviations are also referred to as chat acronyms.
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NetLingo Classification: Acronyms and Text Message