filthy

1. Covered or smeared with filth; disgustingly dirty. See synonyms at dirty.
2. Obscene; scatological.
3. Vile; nasty: a filthy traitor.

adjective

1. Covered or stained with or as if with dirt or other impurities: black, dirty, grimy, grubby, smutty, soiled, unclean, uncleanly. See clean/dirty.
2. Heavily soiled; very dirty or unclean: foul, nasty, squalid, vile. See clean/dirty.
3. Offensive to accepted standards of decency: barnyard, bawdy, broad, coarse, dirty, Fescennine, foul, gross, lewd, nasty, obscene, profane, ribald, scatologic, scatological, scurrilous, smutty, vulgar. Slang raunchy. See decent/indecent.
4. So objectionable as to elicit despisal or deserve condemnation: abhorrent, abominable, antipathetic, contemptible, despicable, despisable, detestable, disgusting, foul, infamous, loathsome, lousy, low, mean², nasty, nefarious, obnoxious, odious, repugnant, rotten, shabby, vile, wretched. See good/bad.

Definition: dirty, polluted
Antonyms: clean, pure, sterile, unpolluted

adj

Definition: vulgar, obscene
Antonyms: chaste, clean, decent, pure

The adjective has 4 meanings:

Meaning #1: disgustingly dirty; filled or smeared with offensive matter
  Synonyms: foul, nasty

Meaning #2: vile; despicable
  Synonyms: dirty, lousy

Meaning #3: (informal) thoroughly unpleasant
  Synonyms: foul, nasty, vile

Meaning #4: characterized by obscenity
  Synonyms: foul, nasty, smutty