nasty: [14] Nasty, now such a widespread term of disapproval, is not that ancient a word in English, and it is not too certain where it came from. In the 14th and 15th centuries it was often spelled naxty, and this, together with one early 17th-century example of nasky, has suggested some connection with Swedish dialect naskug ‘dirty, nasty’. And a link has also been proposed with Dutch nestig ‘dirty’, which may denote etymologically ‘made dirty like a bird’s nest’. ‘Dirty’ was the original sense of the English adjective; the more general ‘unpleasant’ did not begin to emerge until the end of the 17th century.
nasty (adj.)
c. 1400, "foul, filthy, dirty, unclean," of unknown origin; perhaps [Barnhart] from Old French nastre "miserly, envious, malicious, spiteful," shortened form of villenastre "infamous, bad," from vilein "villain" + -astre, pejorative suffix, from Latin -aster.
Alternative etymology [OED] is from Dutch nestig "dirty," literally "like a bird's nest." Likely reinforced in either case by a Scandinavian source (compare Swedish dialectal naskug "dirty, nasty"), which also might be the source of the Middle English word. Of weather, from 1630s; of things generally, "unpleasant, offensive," from 1705. Of people, "ill-tempered," from 1825. Noun meaning "something nasty" is from 1935. Related: Nastily; nastiness.