behest (n.)

c. 1200, from Old English behæs "a vow," perhaps from behatan "to promise" (from be- + hatan "command, call;" see cite) 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 *haissiz, from an extended, suffixed form of PIE root *keie- "to set in motion."