"action of holding back (action or motion); that which restrains, a check, hindrance," early 15c., restreinte, from Old French restreinte, noun use of fem. past participle of restraindre (see restrain).
Specifically in reference to refractory prisoners or dangerous lunatics by 1829. The sense of "reserve, repression of extravagance in manner or style" is from c. 1600. Phrase restraint of trade is by 1630s.
Wherever thought is wholly wanting, or the power to act or forbear according to the direction of thought ; there necessity takes place. This, in an agent capable of volition, when the beginning or continuation of any action is contrary to that preference of his mind, is called compulsion ; when the hindering or stopping any action is contrary to his volition, it is called restraint. [Locke, "Of Human Understanding"]