once: [12] Once originated as the genitive form of one (the genitive case was widely used in Old and Middle English for making adverbs out of nouns – other examples include always, needs, nowadays, and towards). To begin with, this was clearly indicated by its spelling – ones – but from about the start of the 16th century -es was gradually replaced by -ce (reflecting the fact that once retained a voiceless /s/ at its end, whereas in ones it had been voiced to /z/). => one
