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.
He felt deeply ashamed of his own failure.
With a feeling of mortification, I admitted my mistake.
ashamed deep down; inwardly mortified; blushing with shame at a memory
Rare kanji spelling; almost always used in the fixed expression 忸怩たる思い.
Kana spelling is more common in modern writing, especially when the kanji are obscure.
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.