Derived from "individual needs child", a cruel schoolyard insult. Not at all politically correct. Someone who's "not quite normal", an idiot, spaz, etc. ...Then there was a writer in The Guardian in 1987 who said "I first started using the term ‘divvy’ some 20 years ago... When I was growing up in Liverpool in the 1960's it was commonly assumed to be derived from the word ‘individual’", which would seem to support the Urban Dictionary's third attempt.
Which, I admit, made me laugh, recalling an earlier conversation we had here. And on that note, I will say to Noel that I've been reading Steven Pinker's The Stuff of Thought, and while much of it so far is indeed probably only of great interest to linguists, I found the section on linguistic determinism, starting on page 124 of my copy, to be pretty interesting (and relevant to the perennial issue of politically-motivated language policing).