to be sexually unfaithful: to have sex outside marriage or to be a prostitute, and by extension to betray God the way an unfaithful wife betrays her husband
At ground level this word names a woman's sexual activity outside the bonds of marriage: a prostitute selling sex, a bride who was not a virgin, a daughter pushed into the trade. It is the standard Hebrew word for prostitution, and one of its commonest forms is the plain noun built from it, the woman herself, the harlot.
The prophets seized on that ordinary, shame-laden word and turned it into the Bible's sharpest picture of unfaithfulness to God. Israel bound to the Lord is like a wife bound to her husband, so when Israel chases other gods she is said to play the harlot after them. The same word carries both: the woman in the street and the nation that has broken faith. They are not two different words but one image stretched from the bedroom to the covenant.
The literal and the figurative bleed into each other on purpose. When the people worship at the high places, give their loyalty to foreign powers, and join the rites of other gods, the prophets refuse to call it anything cleaner than harlotry. Hosea acts it out by marrying an unfaithful woman; Jeremiah doubles the picture into two faithless sisters; Ezekiel stretches it into a whole life story of betrayal. The force of the word is that it makes unfaithfulness to God feel as concrete, as raw, and as personal as a marriage torn apart.