I've been using this word for years, which most people pronounce "renig." You know, it's when someone makes an agreement and then backs out of it at the time they need to fulfill their part of the bargain. I have hardly ever written it, and the few times I've tried in the past few years, the spell checker tells me it's wrong and I don't get a suggested word.
I thought maybe it derived from the word "renegotiate," but that would only work if both parties wanted to change the terms of the original agreement. If one person backs out when it's time to pay up, that's not renegotiating--it's welching on the deal. (To "welch" means to cheat by avoiding paying a gambling debt, and that can also be spelled "welsh.")
It actually comes from the Latin renegare, "to deny." The correct spelling is "renege" and the past tense is "reneged," which is the form nearly always used, since we don't find out someone's going to renege until they've actually done it.
I'm not sure it matters. Even when I spell it correctly, it looks wrong.