mid-14c., restreinen, "to stop, prevent, curb" (a vice, purpose, appetite, desire), from stem of Old French restraindre, restreindre "to press, push together; curb, bridle; bandage" (12c.), from Latin restringere "draw back tightly, tie back; confine, check" (see restriction).
From late 14c. as "keep (someone or something) from a course of action," hence "keep in check or under control, deprive (someone) of liberty by restraint" (1520s). Related: Restrained; restraining; restrainer; restrainable.
That which we restrain we keep within limits; that which we restrict we keep within certain definite limits; that which we repress we try to put out of existence. [Century Dictionary, 1902]