flashlight n.

Middle English flashen, flasken (c. 1200), "sprinkle or splash (water, powder, etc.); to gush forth;" probably at least partly imitative (compare splash, dash), or in part from PIE *bhleu- "to swell, well up," extended form of root *bhel- (2) "to blow, swell." From c. 1400, of birds, "to dart or flit" also, of fire, "burst into flames." Some of the extended senses perhaps are from Scandinavian. Meanings "burst suddenly into view" (intransitive) and "emit or send forth suddenly" (transitive) are from 1580s. the Sense of "expose the genitals" is recorded by 1846. Related: Flashed; flashing. Flash card is from 1923.

"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.

mid-13c., from Old French torche "torch," also "handful of straw" (for wiping or cleaning, hence French torcher "to wipe, wipe down"), originally "twisted thing," then "torch formed of twisted tow dipped in wax," probably from Vulgar Latin *torca, alteration of Late Latin torqua, from Latin torquere "to twist" (from PIE root *terkw- "to twist").

In Britain, also applied to the battery-driven version (in U.S., a flashlight). To pass the torch is an ancient metaphor from the Greek torch-races (lampadedromia) where the goal was to reach the finish line with the torch still burning. Torch-bearer "leader of a cause" is from 1530s. Torch song is 1927 ("My Melancholy Baby," performed by Tommy Lyman, is said to have been the first so called), from carry a torch "suffer an unrequited love" (also 1927), Broadway slang, but the sense is obscure.