When my dd was about 18 months old we were having trouble with a boy racer across the road, always blasting out loud music, revving up his car for an hour at a time and the usual sort of petty but annoying things that teenage drivers do. He was excelling himself one afternoon while I'd been trying to feed dd, and was still at it while I was trying to get her to have a nap, and I muttered under my breath, "Moron!". I said it once, I have never, ever said it again ... but she called him Moron (despite me telling her it's not a nice word, and she shouldn't use it) for years.
Which was OK except for the time when his brother was out washing his car, and dd (aged about 3) wandered over and asked, very loudly and very clearly, "What are you washing Moron's car for? Can't Moron do it himself?"