to act treacherously, to break faith with someone who trusted you.
At its heart this verb names one thing: betraying a bond. It is not ordinary wrongdoing or simple dishonesty. It is the kind of harm that is possible only because someone first trusted you, so the injury lands inside a relationship that was supposed to hold. A spouse, a friend, a covenant partner, the LORD himself: the word always pictures one party letting another down after faith had been pledged or assumed.
Because the betrayal can happen in any close bond, the same word reaches across several settings. It is what a husband or wife does when the marriage is broken from within. It is what a friend or relative does when loyalty fails just when it is needed. Above all it is what Israel does to God when it turns to other gods, since the prophets picture that turning as a wife abandoning her husband. The shared thread is never just the bad act; it is the trust that was violated to do it.
In the wisdom writings and the prophets the participle hardens into a label for a whole type of person: the treacherous, set against the upright and the faithful. Here the word is less about a single betrayal and more about a settled character, the person whose dealings cannot be relied on. Translations reach for many English words for all of this (deal treacherously, be unfaithful, betray, deal deceitfully), but underneath them stands the one idea of broken faith.