Words related to lighten
"not heavy, having little actual weight," from Old English leoht (West Saxon), leht (Anglian), "not heavy, light in weight; lightly constructed; easy to do, trifling; quick, agile," also of food, sleep, etc., from Proto-Germanic *lingkhtaz (source also of Old Norse lettr, Swedish lätt, Old Frisian, Middle Dutch licht, German leicht, Gothic leihts), from PIE root *legwh- "not heavy, having little weight." The adverb is Old English leohte, from the adjective.
Meaning "frivolous" is from early 13c.; that of "unchaste" from late 14c., both from the notion of "lacking moral gravity" (compare levity). Of literature from 1590s. Light industry (1919) makes use of relatively lightweight materials. The notion in make light of (1520s) is "unimportance." Alternative spelling lite, the darling of advertisers, is first recorded 1962. Light horse "light armed cavalry" is from 1530s. Light-skirts "woman of easy virtue" is attested from 1590s. Lighter-than-air (adj.) is from 1887.
word-forming element making verbs (such as darken, weaken) from adjectives or nouns, from Old English -nian, from Proto-Germanic *-inojan (also source of Old Norse -na), from PIE adjectival suffix *-no-. Most active in Middle English and early modern English, hence most verbs in -en are comparatively recent.
"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.
late 14c., "to remove the dimness or blindness" (usually figurative, from one's eyes or heart); see en- (1) + lighten (v.2). From 1660s as "supply with intellectual light." Literal senses are later and less common in English: "put light in" is from 1580s; "shed light upon" is from 1610s. Related: Enlightened; enlightening. Old English had inlihtan "to illuminate, enlighten."