foreignness: what belongs to another people or land, and so is alien to Israel and its God
This is an abstract word for foreignness, the quality of coming from somewhere else and belonging to another people. It almost never stands alone. Instead it hangs off another word in fixed phrases: a "son of foreignness" is a foreigner, the "gods of foreignness" are foreign gods, and the "soil of foreignness" is a foreign land. In each case the word marks something as not one's own, as belonging to an outside allegiance.
The force of the word is not merely geographic. To call someone or something foreign in this sense is to set it over against Israel and against the LORD. That is why the same word covers a foreign person, a foreign god, and a foreign country: all three name a world outside the covenant, whether that outsider is a threat, a temptation, or simply a stranger far from home.
Because the word is used mostly of gods that Israel is told to put away, it often carries a warning. Yet the range is wide enough that a foreigner who turns to the LORD can also be named with it, as in Isaiah, where the outsider who joins himself to God is welcomed rather than shut out.