slight adj.

early 14c., "flat, smooth, sleek; hairless," originally northern according to OED and probably from a Scandinavian source akin to Old Norse slettr "smooth, sleek," from Proto-Germanic *slikhtaz "slippery; flat, level, plain" (source also of Old Saxon slicht; Old Frisian sliucht, Low German slicht "smooth, plain common;" Old English -sliht "level," attested in eorðslihtes "level with the ground;" Old Frisian sliucht "smooth, slight," Middle Dutch sleht "even, plain," Old High German sleht, Gothic slaihts "smooth"), probably from a collateral form of PIE *sleig- "to smooth, glide, be muddy," from root *(s)lei- "slimy" (see slime (n.), and compare slick (v.)).

The original sense is obsolete. The evolution probably is from "smooth" (c. 1300), to "slim, slender; of light texture," hence "not good or strong; insubstantial, trifling, inferior, insignificant" (early 14c.). The meaning "small in amount" is from 1520s.

The sense of German cognate schlecht developed from "smooth, plain, simple" to "bad, mean, base," and as it did it was replaced in the original senses by schlicht, a back-formation from schlichten "to smooth, to plane," a derivative of schlecht in the old sense [Klein]. English slight also had the same tendency, and 15c.-17c., used of persons, could mean "humble, low; of little worth or account."

c. 1300, slighten, "make plain or smooth with a sleek-stone," from slight (adj.); hence also "adorn oneself" (early 14c.) and in 17c. often "raze, level" (fortifications, cities, etc.). These senses are obsolete. The meaning "treat with indifference" (1590s) is from the adjective in the sense of "having little worth." Related: Slighted; slighting. As a verb the Elizabethan dramatists also used slighten.

1550s, "small amount or weight" (a sense now obsolete), from slight (v.). The meaning "intentional neglect or ignoring out of displeasure or contempt" is from 1701, probably via 17c.-18c. phrase make (a) slight of "regard as trifling or unimportant."