denotes "unclean, impure" (a, negative, and kathairo, "to purify"), (a) ceremonially, e.g., Acts 10:14,28; (b) morally, always, in the Gospels, of unclean spirits; it is translated "foul" in the AV of Mark 9:25; Revelation 18:2 , but always "unclean" in the RV. Since the word primarily had a ceremonial significance, the moral significance is less prominent as applied to a spirit, than when poneros, "wicked," is so applied. Cp. akatharsia, "uncleanness." See UNCLEAN.

Note: In Revelation 17:4 the best mss. have this word in the plural, RV, "the unclean things" (akathartes, "filthiness," in some mss.).

Bibliography Information
Vines, W. E., M. A. Entry for 'Foul'. Vine's Expository Dictionary of NT Words. https://www.studylight.org/dictionaries/eng/ved/f/foul.html. 1940.

Bible Dictionaries Foul

Vine's Expository Dictionary of NT Words

Bible Dictionaries
Foul