The word "后悔不迭" is formed by combining the compound "后悔", meaning "to regret", with the adverbial phrase "不迭", which denotes "incessantly" or "without ceasing". Here, "后" contributes a sense of "after" or "later", and "悔" means "regret", together conveying the idea of regret after an event. "不" negates, and "迭" implies repetition or alternation, so "不迭" emphasizes unstopping action. When merged, the phrase intensifies "后悔" to express persistent, overwhelming regret, structurally creating a fixed expression where the modifier "不迭" amplifies the preceding verb.