The story
A variant of OMG, presumably from excited users accidentally hitting the Z key while attempting to press shift, or from conscious imitation of such excitement. The term is said to have been coined (likely in the early 2000s) by a Something Awful user named “Rick_Feynman”, in an ICQ chat between him and fellow user “Hannibal” (see also his Usenet message quoted below), though it was already in use before that. Usage of the term on the boards of Something Awful and 4chan has greatly contributed to its popularization.
Etymology from Wiktionary, CC BY-SA 4.0.