Old English settan (transitive) "cause to sit, put in some place, fix firmly; build, found; appoint, assign," from Proto-Germanic *(bi)satjan "to cause to sit, set" (cf. Old Norse setja, Swedish sätta, Old Saxon settian, Old Frisian setta, Dutch zetten, German setzen, Gothic satjan), causative form of PIE *sod-, variant of *sed- "to sit" (see sit (v.)). Also cf. set (n.2).
Intransitive sense from c.1200, "be seated." Used in many disparate senses by Middle English; sense of "make or cause to do, act, or be; start" and that of "mount a gemstone" attested by mid-13c. Confused with sit since early 14c. Of the sun, moon, etc., "to go down," recorded from c.1300, perhaps from similar use of the cognates in Scandinavian languages. To set (something) on "incite to attack" (c.1300) originally was in reference to hounds and game.