Old English styrian "to stir, move; rouse, agitate, incite, urge" (transitive and intransitive), from Proto-Germanic *sturjan (source also of Middle Dutch stoeren, Dutch storen "to disturb," Old High German storan "to scatter, destroy," German stören "to disturb"), from PIE *(s)twer- (1) "to turn, whirl" (see storm (n.)). Related: Stirred; stirring. Stir-fry (v.) is attested from 1959.
"commotion, disturbance, tumult," late 14c. (in phrase on steir), probably from a Scandinavian source, such as Old Norse styrr "disturbance, tumult," from the same root as stir (v.)). The sense of "movement, bustle" (1560s) probably is from the English verb.