The English adverb "once," meaning "on one occasion or for one time only," as well as "at some time in the past" or "formerly," traces its origins to Old English and Middle English linguistic developments. Its etymology reveals a clear connection to the numeral "one," combined with a genitive suffix that marks it as an adverbial expression of time.
The earliest form of "once" appears in Old English as "ānes," which is the genitive singular of "ān," the Old English word for "one." The root "ān" itself is inherited from the Proto-Germanic *ainaz, which is the source of cognates in other Germanic languages such as Old High German "ein," Old Norse "einn," and Gothic "ains," all meaning "one." This root ultimately descends from the Proto-Indo-European *óynos or *óin-, though the precise form and phonetic details remain somewhat uncertain due to the nature of reconstructed proto-languages.
In Old English, the genitive singular suffix "-es" was commonly used not only to indicate possession but also to form adverbs expressing time or manner. This genitive suffix is evident in several adverbs formed from numerals or other nouns, such as "twice" (from "twīwa" or "twīes"), "always" (from "eal" meaning "all" plus "-wes" or "-ways"), "sometimes" (from "sum" meaning "some" plus "-times"), and "besides" (from "side" plus "-es"). The suffix in these cases does not mark plurality but rather an adverbial genitive, effectively meaning "of one (time)" or "on one occasion" in the case of "ānes."
During the Middle English period, roughly from the 12th to the 15th century, the Old English "ānes" evolved into "ones." This form retained the genitive sense but began to be spelled with an "o" instead of the Old English "ā," reflecting changes in vowel pronunciation and orthographic conventions. The Middle English "ones" was pronounced with a final /s/ sound, consistent with the genitive suffix.
By the 15th century, the spelling of "ones" shifted to "once." The introduction of the letter "c" in the spelling was not arbitrary but served a phonetic and orthographic purpose. The "c" represented the /s/ sound, a common practice in Middle English to distinguish certain sibilant sounds and to differentiate words that were otherwise homographs. In this case, the spelling "once" helped to distinguish the adverb from the plural pronoun "ones," which continued
It is important to note that "once" is an inherited word from Old English and is not a borrowing from another language. Its formation is a clear example of the use of the genitive singular suffix to create adverbs of time, a morphological pattern native to Old English and other Germanic languages. The semantic development from "of one (time)" to the adverbial sense "on one occasion" is straightforward and transparent.
In summary, "once" originates from the Old English "ānes," the genitive singular of "ān" meaning "one," combined with the genitive suffix "-es" used adverbially. This form passed into Middle English as "ones" and later acquired the spelling "once" in the 15th century to reflect pronunciation and to distinguish it from the plural "ones." The word is an inherited Germanic term, with no known borrowing influences, and its evolution exemplifies the morphological and orthographic changes characteristic of English from the Old to the Middle English periods.