I'm fascinated by words and as I thought about the steadiness of the candle flame I wondered where the word came from and about it's meaning. I got this from an etymology site:
steady(adj.)1520s, "firmly fixed in place or station" (displacing earlier steadfast, which came to be used of persons and characters), from stead + adjectival suffix -y (2), perhaps on model of Middle Dutch, Middle Low German stadig.