sight (n.) Look up sight at Dictionary.com
Old English gesiht, gesihð "thing seen," from Proto-Germanic *sekh(w)- (cf. Danish sigte, Swedish sigt, Middle Dutch sicht, Dutch zicht, Old High German siht, German Sicht, Gesicht), stem of Old English seon (see see (v.)). Meaning "perception or apprehension by means of the eyes" is from early 13c.
Verily, truth is sight. Therefore if two people should come disputing, saying, 'I have seen,' 'I have heard,' we should trust the one who says 'I have seen.' [Brhadaranyaka Upanishad 5.14.4]
Meaning "device on a firearm to assist in aiming" is from 1580s; the verb in this sense is from 1842. Related: Sighted; sighting. Sight for sore eyes "welcome visitor" is attested from 1738; sight unseen "without previous inspection" is from 1892. Sight gag first attested 1944.