I recall it starting in 2013 with a mispronunciation by professor Mark T Williams of Boston University. It led to him being known as Professor Bitcorn. I haven't found the actual quotation yet but I'm sure it's out there somewhere. For now I've had my piss and re-hydration and I'm going back to bed.
https://en.bitcoin.it/wiki/Mark_T._WilliamsI checked the source of the above image: It's user
bitcorn, registered in 2011.
That user may have used the word in 2011 but it was Mark T Williams use that popularized its use.