Contrary to whatever your reading the original word came from modern poker. The word is a combination of the 2 root words "Ridiculous" and "Donkey". The ridiculous comes for the bad judgment at a poker table and the Donkey is what poker players call other poker players that apply bad judgment. Hence "redonkulous" ridiculous donkey.
Why would call that bet with a 3 outer? That's redonkulous.