Behest is an ancient word: it is almost a thousand years old. It was formed from the prefix be- and the verb hātan ("to command" or "to promise"), and its Old English ancestor was used exclusively in the sense of "promise," a now-obsolete meaning that continued on in Middle English especially in the phrase "the land of behest." The "command" meaning of behest is also ancient but it's still in good use, typically referring to an authoritative order. Behest is now also used with a less weighty meaning; it can refer to an urgent prompting, as in "a repeat performance at the behest of the troupe's fans."