What is the X in X-mas?
Old English *fyxen (implied in adjective fyxan), fem. of fox (see fox, and cf. Middle High German vühsinne, German füchsin). Solitary English survival of the Germanic feminine suffix -en, -in (cf. Old English gyden "goddess;" mynecen "nun," from munuc "monk;" wlyfen "she-wolf"). The figurative sense "ill-tempered woman" is attested from 1570s. The spelling shift from -f- to -v- began late 1500s (see V).