deep shame; strong embarrassment (over one's own actions)
Formal/literary expression for a lingering, painful sense of shame about one's own conduct. Often used in reflective or confessional contexts.
He looked back on his slip of the tongue with deep shame.
あの時の判断ミスは、今でも忸怩たる思いだ。
I still feel deeply ashamed about that misjudgment back then.
後悔 is general regret, which may not involve shame. 忸怩たる思い specifically emphasizes a painful, self-critical shame.
Standard kanji spelling; 忸怩 is a rare kanji compound meaning 'ashamed'.
Kana spelling is common when the kanji are considered difficult or obscure.
From the classical Chinese-derived compound 忸怩 (jikuji, 'ashamed') + the classical auxiliary たる (taru, attributive form of たり) + 思い (omoi, 'feeling'). The phrase is a set literary expression.