This is the 8409th most frequent Japanese word.
悔し
"悔し" (くやし, kuyashi) means "regrettable" or "frustrating."
悔しい is used to express a feeling of regret or frustration due to an undesirable outcome.
試合に負けてとても悔しい気持ちです。
I am feeling very regretful after losing the match.
悔しい here indicates a sense of envy mixed with regret for not achieving a similar outcome.
彼女の成功を聞いて、少し悔しい思いを抱いてしまった。
Listening about her success, I felt a slight bitter feeling.
悔しい is utilized to denote frustration and disappointment when hard work does not lead to success.