IPA: //ˈfɪl.θi.ər//
KK: /ˈfɪlθiər/
More dirty or unclean than something else.
The bathroom was filthier than I expected after the party.
Filthier → It is formed from "filth" (from Old English "fylð", meaning dirt or uncleanliness) and the comparative suffix "-ier" (meaning more). The word "filthier" means more dirty or more unclean than something else.
Think of 'filth' as representing dirtiness, and the '-ier' ending indicates a comparison, so 'filthier' means something is more dirty than another.