忿 (fèn) is a bound form (b.f.) meaning 'anger' or 'indignation'. It rarely appears alone in modern Chinese — you'll find it in compounds like 忿怒 (fèn nù, 'rage'), 忿恨 (fèn hèn, 'resentment'), or the chengyu 忿忿不平 (fèn fèn bù píng, 'indignant and aggrieved'). As a bound form, it must attach to another character to form a word; you cannot use it as a standalone noun or verb like 生气.