WordNet sense 1 (slanderous defamation):

vilification, malignment

WordNet sense 2 (a thin tissue or blood sample spread on a glass slide and stained for cytologic examination and diagnosis under a microscope):

cytosmear, cytologic smear

WordNet sense 3 (a blemish made by dirt):

blot, daub, slur, smirch, smudge, spot

WordNet sense 4 (make a smudge on; soil by smudging):

blur, smutch

WordNet sense 5 (charge falsely or with malicious intent; attack the good name and reputation of someone):

besmirch, calumniate, defame, denigrate, slander, sully, asperse

WordNet sense 6 (an act that brings discredit to the person who does it):

stain