The Hebrew verb naaph means to commit adultery. Used about 31 times in the OT, it refers to sexual unfaithfulness within marriage. Crucially, the prophets also use it metaphorically to describe Israel's spiritual unfaithfulness to God — pursuing other gods is described as adultery against the divine covenant.
Naaph is prohibited in the Seventh Commandment (Exodus 20:14) and represents a fundamental breach of the covenant bond — both marital and spiritual. The prophets Hosea, Jeremiah, and Ezekiel extensively use this metaphor: Israel has committed spiritual naaph by worshipping idols. This illuminates why marriage is so sacred in Scripture — it reflects the covenant faithfulness between God and His people. Jesus deepens this in Matthew 5:27–28 by extending it to the level of the heart.