taru-adjective, adverb taking the 'to' particle
discontented; unhappy; dissatisfied
Literary and somewhat archaic; describes a state of being discontented or resentful, often used with として or たる in classical or formal writing.
彼は怏々として楽しまない様子だった。
He seemed discontented and not enjoying himself.
その知らせを聞いて、一同は怏々たる思いだった。
Upon hearing the news, everyone felt deeply dissatisfied.
The exact derivation is uncertain; the kanji 怏 and 鞅 both carry meanings related to dissatisfaction or resentment, and the reduplicated form creates a literary adjective/adverb.