Vixen
late Middle English fixen, perhaps from the Old English adjective fyxen ‘of a fox’. The v- is from the form of the word in southern English dialect.
wiktionary
Alteration of earlier fixen, from Middle English fixen, from Old English fyxen, from Proto-West Germanic *fuhsini; synchronically analyzable as fox + -en. Voiced v- is from the Southern dialectal forms of Middle English.
etymonline
vixen (n.)
Old English *fyxen (implied in adjective fyxan), fem. of fox (see fox (n.) and cognate with Middle High German vühsinne, German füchsin). Solitary English survival of the Germanic feminine suffix -en, -in (also in Old English gyden "goddess;" mynecen "nun," from munuc "monk;" wlyfen "she-wolf," etc.). The figurative sense "ill-tempered woman" is attested from 1570s. The spelling shift from -f- to -v- began late 1500s (see V).
