【Dirty】 , 【filthy】 , 【foul】 , 【nasty】 , 【squalid】 mean conspicuously unclean or impure.

【Dirty】 is the general term for what is sullied or defiled with dirt of any kind.

【Filthy】 is a much stronger term than  【dirty】 in its suggestion of offensiveness; it often suggests gradually accumulated dirt which besmears or begrimes rather than merely soils.

【Foul】 carries a still stronger implication of revolting offensiveness; it often implies an unwholesome or malodorous state resulting from the decay of putrescible matter. It may come near to loathsome or disgusting.

【Nasty】 applies chiefly to what is repugnant to a person who is fastidious about cleanliness.

Sometimes 【nasty】 is softened to a mere synonym for "objectionable, disagreeable".

【Squalid】  adds to the idea of dirtiness or filth that of extreme slovenliness or neglect.

All of these terms may imply moral uncleanness or baseness or obscenity.

【Dirty】 , however, stresses meanness or despicability.

【Filthy】 and 【foul】 imply disgusting obscenity,  【filthy】 stressing the presence of obscenity and 【foul】 , its ugliness.

【Nasty】 implies a peculiarly offensive unpleasantness.

【Squalid】 implies sordidness as well as baseness.

The first four terms are used also of weather, meaning the opposite of clear and thereby implying rainy, snowy, stormy, or foggy weather.

【Soil】 , 【dirty】 ,  【sully】 ,  【tarnish】 ,  【foul】 ,  【befoul】 ,  【smirch】 ,  【besmirch】 ,  【grime】 ,  【begrime】  can all mean to make or become unclean.

【Soil】  basically implies fundamental defilement or pollution (as of the mind or spirit) <why war soils and disarranges whatever it touches, I cannot say —Kenneth Roberts >  <making that room our Chapter, our one mind where all that this world soiled should be refined —Masefield >  but in much of its use it applies to a making or becoming superficially and literally unclean (as by spotting or staining or smudging).

In this sense the word is very close to the corresponding sense of  【dirty】 , which is slightly stronger in its implication of uncleanliness and especially of disagreeable uncleanliness; thus, "to  【soil】  one’s clothes" may merely imply that the freshness of a clean or new thing is lost, but "to  【dirty】  one’s clothes" usually implies some activity which has plainly left its unclean traces upon the garments. In its extended use, too,  【dirty】 tends to stress the unpleasant effect and typically suggests a making squalid or nasty of something that in itself is normal, wholesome, or clean.

【Sully】 implies the staining or soiling of something that is pure, fresh, limpid, or innocent.

It is used more often in reference to immaterial or spiritual than to physical soiling.

【Tarnish】 basically implies the dulling or dimming of the luster of a thing by chemical action (as of air, dust, or dirt).

In extended use it suggests a dimming rather than a total sullying of something of value.

【Foul】  and the intensive form  【befoul】 stress a making filthy or nasty and apply either to a material or an immaterial thing. They often suggest pollution or defiling by something highly offensive or disagreeable.

【Smirch】  and the intensive form  【besmirch】  may emphasize a discoloring by or as if by soot, smoke, or mud; usually they come close to  【sully】 in implying a destruction of immaculateness, but they seldom carry as clear an implication of an effect on real virtue or purity as they do of a darkening or blackening of appearance, reputation, honor, or good name.

【Grime】  and the more usual  【begrime】  intensify the meaning of  【dirty】 and typically suggest deeply imbedded dirt often accumulated over a prolonged period.