nasty (adj.)

late 14c., nasti, "foul, filthy, dirty, unclean," literally or figuratively, a word of uncertain origin. Middle English Compendium says from Old Norse (compare Swedish dialectal and Danish naskug, nasket "dirty, nasty") with Middle English adjectival suffix -i. There was a variant nasky in early Modern English.

Barnhart suggests Old French nastre "miserly, envious, malicious, spiteful," shortened form of villenastre "infamous, bad," from vilain "villain" (see villain) + -astre, pejorative suffix, from Latin -aster. Another alternative etymology [mentioned in OED] is from Dutch nestig "dirty," literally "like a bird's nest."

From c. 1600 as "indecent, obscene" ("morally filthy"). Of weather, "foul, stormy," from 1630s; of things generally, "unpleasant, offensive; troublesome, annoying," from 1705. Of people, "ill-tempered, mean," from 1825. The noun meaning "something nasty" is from 1935. Related: Nastily; nastiness.

Nasty, in England frequently meaning ill-tempered or cross-grained ( Slang Dictionary, p. 186), and in this sense admitted into good society, denotes in America something disgusting in point of smell, taste, or even moral character, and is not considered a proper word to be used in the presence of ladies. [M. Schele De Vere, "Americanisms," 1872]