It should first be noted that the term is repeated (three times) and so John draws our attention to it as an important concept in the context. In the Old Testament, the Lord gave his people Israel purity laws because anything impure/unclean is at odds with the Lord’s identity. So, anyone unclean could not participate in the ceremonies occurring at the tabernacle (Leviticus 7:19–20). Through laws pertaining to diet, childbirth, illness, and bodily discharge (Leviticus 11—15), the Lord impressed on his people that there was nothing unclean or impure about him, a reality his people needed to embrace and reflect in daily life. That meant specifically that the people of Israel were not to eat those specific animals or birds or fish or reptiles that God had declared unclean (Leviticus 11), and when an Israelite became unclean (e.g., through discharge, Leviticus 15), he had to subject himself to a particular ritual and make a sacrifice to become ceremonially clean again. Conversely, when the prophets announced the fall of Israel’s enemies, they pictured their ruins as inhabited by creatures God had declared unclean (Isaiah 13:21–22; Jeremiah 51:37). Characteristic of many of these unclean creatures was that they feasted on carrion. This latter characteristic also explains why such beasts are “detestable.”
2 And he called out with a mighty voice, “Fallen, fallen is Babylon the great! She has become a dwelling place for demons, a haunt for every unclean spirit, a haunt for every unclean bird, a haunt for every unclean and detestable beast.