filthy (adj.)

late 12c., fulthe, "corrupt, sinful," from filth + -y (2). Meaning "physically unclean, dirty, noisome" is from late 14c. Meaning "morally dirty, obscene" is from 1530s.

In early use often hardly more emphatic than the mod. dirty; it is now a violent expression of disgust, seldom employed in polite colloquial speech. [OED]

Related: Filthily; filthiness.

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Definitions of filthy from WordNet

filthy (adj.)
disgustingly dirty; filled or smeared with offensive matter;
as filthy as a pigsty
Synonyms: foul / nasty
filthy (adj.)
vile; despicable;
a filthy traitor
Synonyms: dirty / lousy
filthy (adj.)
characterized by obscenity;
had a filthy mouth
Synonyms: cruddy / foul / nasty / smutty