c. 1200, from Old English behæs "a vow," perhaps from behatan "to promise" (from be- + hatan "command, call") and confused with obsolete hest "command," which may account for the unetymological -t as well as the Middle English shift in meaning to "command, injunction" (late 12c.). Both hatan and hest are from Proto-Germanic *haitanan, for which see hight.
