This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
憠 combines 厥 (possibly phonetic) with 心 (heart/mind). The exact historical development is uncertain, and no clear meaning is recorded.
With 心 (heart) under 厥, imagine a strong, stubborn heart that refuses to yield. The reading つよ.い suggests strength, so picture a heart so strong it overpowers everything.
For ケツ, imagine a strong heart that never gets tired, like a kettle that keeps boiling: kettle -> ケツ, and the heart stays strong.