promiscuous sexual immorality, used above all as a picture of unfaithfulness to God
At its root this word means promiscuity: giving oneself sexually where one has no right to. But in the Hebrew Bible it is almost never a report of ordinary sexual sin. The prophets seize the image and turn it on the nation: Israel bound to God like a wife to a husband, then chasing after other gods like a woman chasing other men.
So the word carries two layers at once. On the surface is the physical act of harlotry; underneath, and usually in view, is the deeper betrayal it stands for, worship of idols and reliance on foreign powers instead of trust in God. When Jeremiah says the land was polluted with her whoredom, or Ezekiel condemns a whoredom carried all the way from Egypt, the sexual language is doing the work of an accusation about loyalty.
The same word therefore reaches from a body to a covenant. It is close kin to the verb זָנָה, "to play the harlot," and to two other nouns for the same failure, תַּזְנוּת and זְנוּנִים; the prophets also set it beside נִאֻף, "adultery," the breaking of a marriage already made.