taru-adjective, adverb taking the 'to' particle
ashamed; mortified; deeply embarrassed
Literary and somewhat archaic; almost exclusively used in the set phrase 忸怩たる思い (じくじたるおもい), meaning a feeling of deep shame or mortification. Rarely used outside this expression.
彼は自分の失敗に忸怩たる思いを抱いた。
He felt deeply ashamed of his own failure.
忸怩たる思いで過ちを認めた。
With a feeling of mortification, I admitted my mistake.
The kanji 忸 and 怩 both relate to shame or embarrassment. The exact historical derivation is uncertain, but the compound is used in classical Chinese and adopted into literary Japanese.