What is the X in X-mas?
suffix attached to verbs to mean their action, result, product, material, etc., from Old English -ing, -ung, from Proto-Germanic *unga (cf. Old Norse -ing, Dutch -ing, German -ung). Originally used to form nouns from verbs and to denote completed or habitual action. Its use has been greatly expanded in Middle and Modern English.
suffix used form the present participle of verbs, from Old English -ende (cf. German -end, Gothic -and, Sanskrit -ant, Greek -on, Latin -ans). It evolved into -ing in 13c.-14c.