There are no direct antonyms for the phrase "plight one's troth", as it is a somewhat archaic expression meaning to pledge one's faithfulness or promise to marry. However, some possible opposite concepts might include breaking a pledge or vow, being unfaithful or disloyal, or renouncing one's commitment. Such antonyms might include terms like forsaking, reneging, betraying, or backtracking. Of course, since the phrase is fairly uncommon and specific, it may not be necessary or useful to find direct antonyms, as different contexts and situations may require different nuances and shades of meaning.
