side-light n.

late Old English, "long, broad, spacious; extending lengthwise," from side (n.). Compare Old Norse siðr "long, hanging down." From late 14c. as "being from or toward the side," hence also "subordinate." Also "apart from the main course" of anything, as in side-road (1854); side-trip (1911). In side-eye (by 1922) the notion is "directed sideways."

"brightness, radiant energy, that which makes things visible," Old English leht (Anglian), leoht (West Saxon), "light, daylight; spiritual illumination," from Proto-Germanic *leukhtam (source also of Old Saxon lioht, Old Frisian liacht, Middle Dutch lucht, Dutch licht, Old High German lioht, German Licht, Gothic liuhaþ "light"), from PIE root *leuk- "light, brightness."

The -gh- was an Anglo-French scribal attempt to render the Germanic hard -h- sound, which has since disappeared from this word. The figurative spiritual sense was in Old English; the sense of "mental illumination" is first recorded mid-15c. Meaning "something used for igniting" is from 1680s. Meaning "a consideration which puts something in a certain view" (as in in light of) is from 1680s. Short for traffic light from 1938. Quaker use is by 1650s; New Light/Old Light in church doctrine also is from 1650s. Meaning "person eminent or conspicuous" is from 1590s. A source of joy or delight has been the light of (someone's) eyes since Old English:

Ðu eart dohtor min, minra eagna leoht [Juliana].

Phrases such as according to (one's) lights "to the best of one's natural or acquired capacities" preserve an older sense attested from 1520s. To figuratively stand in (someone's) light is from late 14c. To see the light "come into the world" is from 1680s; later as "come to full realization" (1812). The rock concert light-show is from 1966. To be out like a light "suddenly or completely unconscious" is from 1934.