Foul - Vine's Expository Dictionary of New Testament Words
Foul[ 1,,G169, akathartos ]
denotes unclean, impure" (a, negative, and kathairo, "to purify"),
(a) ceremonially, e.g., Acts 10:14, Acts 10:28;
(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.).