What is the X in X-mas?
mid-13c., from Old French seisir "to take possession of, take by force; put in possession of, bestow upon" (Modern French saisir), from Late Latin sacire, which is generally held to be from a Germanic source, but the exact origin is uncertain. Perhaps from Frankish *sakjan "lay claim to" (cf. Gothic sokjan, Old English secan "to seek;" see seek). Or perhaps from Proto-Germanic *satjan "to place" (see set (v.)).
Originally a legal term in reference to feudal property holdings or offices. Meaning "to grip with the hands or teeth" is from c.1300; that of "to take possession by force or capture" (of a city, etc.) is from mid-14c. Figurative use, with reference to death, disease, fear, etc. is from late 14c. Meaning "to grasp with the mind" is attested from 1855. Of engines or other mechanisms, attested from 1878. Related: Seized; seizing.