be angry
忿 centers on a state of anger or indignation, with the heart/mind component emphasizing the emotional nature of the feeling.
忿 combines 分 (divide, part) with 心 (heart, mind), suggesting a heart divided or stirred up by anger. The exact historical development is uncertain, but the phono-semantic structure is plausible.
When your heart 心 is divided 分, you feel torn and angry. Picture your heart splitting in two from frustration.
For フン, imagine the fun of a sudden angry outburst: fun -> フン, but here the fun is actually anger boiling over.