sight-reading n.

Middle English sight, from Old English sihð, gesiht, gesihð "thing seen; power or faculty of sight; aspect; vision; apparition," from Proto-Germanic *sekh(w)- (source also of Danish sigte, Swedish sigt, Middle Dutch sicht, Dutch zicht, Old High German siht, German Sicht, Gesicht), stem that also yielded Old English seon (see see (v.)), with noun suffix -th (2), later -t (14c.).

The meaning "perception or apprehension by means of the eyes" is from early 13c. The meaning "device on a firearm to assist in aiming" is from 1580s. A "show" of something, hence, colloquially, "a great many; a lot," (late 14c.). As "something that calls forth glances of shock, amusement, etc., a shocking spectacle," by 1862.

Sight for sore eyes "welcome visitor" is attested from 1738; sight unseen (adv.) "without previous inspection" is from 1892. Sight gag is attested by 1944. To feel or know something at first sight is from c. 1300. From the firearm aiming sense come in (one's) sights; have (one's) sights set on something. To keep out of sight is from late 14c.; to be out of (someone's) sight is from c. 1400.

Middle English reden, ireden, "to counsel, advise," also "to read," from Old English rædan, gerædan (West Saxon), redan, geredan (Anglian) "to advise, counsel, persuade; discuss, deliberate; rule, guide; arrange, equip; forebode; to read (observe and apprehend the meaning of something written), utter aloud (words, letters, etc.); to explain; to learn through reading; to put in order."

This is reconstructed to be from Proto-Germanic *redan, source also of Old Norse raða, Old Frisian reda, Dutch raden, Old High German ratan, German raten "to advise, counsel, interpret, guess," from PIE root *re- "to reason, count."

Cognate words in most modern Germanic languages still mean "counsel, advise" (compare rede). Old English also had a related noun ræd, red "advice," and read is connected to riddle (n.1) via the notion of "interpret." Century Dictionary notes that the past participle should be written red, as it formerly was, and as in lead/led. Middle English past participle variants include eradde, irad, ired, iræd, irudde.  

The sense-transference to "interpret and understand the meaning of written symbols" is said to be unique to English and (perhaps under Old English influence) Old Norse raða. Most languages use a word rooted in the idea of "gather up" as their word for "read" (such as French lire, from Latin legere).

Sense of "make out the character of (a person)" is attested from 1610s. Musical sense of "perform (at first sight) from the notes" is by 1792. To read up "systematically study" is from 1842; read out (v.) "expel by proclamation" (Society of Friends) is from 1788. Read-only in computer jargon is recorded from 1961.