Why is the ninth month called September?
mid-15c., apparently from in + step, "though this hardly makes sense" [Weekley]. An Old English word for "instep" was fotwelm. Middle English also had a verb instep "to track, trace" (c.1400).
instep in·step (ĭn'stěp')n. The arched middle part of the foot between toes and ankle.