The story
An intentionally humorous loose rendering of the otherwise unpronounceable initialism ISHYGDDT (“I seriously hope you guys don't do this”), influenced by expressiveness (comparable to expressions like the bomb diggy) and phonetic harmony; compare the etymology of Humvee /Hummer for the same sort of process. Originated on 4chan circa 2011 before spreading to other online communities.
Etymology from Wiktionary, CC BY-SA 4.0.