In the OT, which was the cultural/spiritual background of Paul, there was ritual impurity and moral impurity. Ritual impurity did not necessarily or automatically mean moral impurity. Many forms of ritual impurity were unavoidable e.g. touching a dead person or animal corpse; others were quite “natural” like menstruation or giving birth.
One synonym for impure is “defiled.” Another is “corrupt.” Both of these words have specific connotations in our cultures and languages - i.e. depending on where we live, these words conjure up different things. But if you go back to the OT here are some scriptures that may show the wider scope of the concept.
“Do not turn to mediums or seek out spiritists, for you will be defiled by them. I am the Lord your God.”
“Say to the Israelites: ‘Any Israelite or any foreigner residing in Israel who sacrifices any of his children to Molek is to be put to death. The members of the community are to stone him. I myself will set my face against him and will cut him off from his people; for by sacrificing his children to Molek, he has defiled my sanctuary and profaned my holy name.
“‘Do not pollute the land where you are. Bloodshed pollutes the land, and atonement cannot be made for the land on which blood has been shed, except by the blood of the one who shed it. Do not defile the land where you live and where I dwell, for I, the Lord, dwell among the Israelites.’”
Although you wash yourself with soap and use an abundance of cleansing powder,
the stain of your guilt is still before me,” declares the Sovereign Lord.
How can you say, ‘I am not defiled; I have not run after the Baals’? (Defilement by idolatry)
“Son of man, when the people of Israel were living in their own land, they defiled it by their conduct and their actions…" (This implies that there were many behavioural patterns that God viewed as impure.)
Morally speaking, any breaking of God’s command (by commission or omission) would be a corruption, a defilement, of our essential being, created in the image of God. There is also a link (see above) to profanity (making unclean) - which we connect to our speech - whether the specific words we use, or the sinful use of our talk in slander and gossip. Like Peter at the last supper, we may have been bathed, figuratively, in the blood of Jesus and therefore clean, but we need “our feet washed” on a very regular basis.