Rare standalone noun; in modern Japanese the concept is more commonly expressed by 悶え (もだえ) or 苦悶 (くもん). This form is mostly seen in literary or classical contexts.
The kanji 悶 combines 門 (gate) and 心 (heart), suggesting a heart confined or in distress. The reading もん is the on'yomi; the standalone noun use is rare and largely literary.
