To be jealous of or envious of someone; to begrudge another's success or possessions. A more literary/formal word; 日常語 often uses 妬む (ねたむ).
彼は友人の出世を心の底から嫉んでいたようだ。
He seemed to be deeply jealous of his friend's promotion.
古い小説では、登場人物が互いに嫉む描写がしばしば見られる。
In older novels, it's common to see characters depicted as envying one another.
彼女は妹の才能を嫉んでしまい、素直に喜べなかった。
She ended up envying her sister's talent and couldn't honestly be happy.