1930s; earliest use found in The Wisconsin State Journal. Probably a humorous alteration of filthy, intended to represent a pronunciation by non-native speakers.
Definition of feelthy in US English:
feelthy
adjectiveˈfiːlθi
slang, humorous
Filthy, indecent, obscene.
Origin
1930s; earliest use found in The Wisconsin State Journal. Probably a humorous alteration of filthy, intended to represent a pronunciation by non-native speakers.