闷

Default reading mèn covers the emotional / sealed-up senses: bored, depressed, gloomy, taciturn, hermetically closed. Showpiece compounds: 郁闷 (gloomy / depressed), 纳闷 (puzzled), 闷闷不乐 (in low spirits), 苦闷 (dejected), 生闷气 (to sulk).

A second reading 闷 (first tone) is the physical 'stuffy / muggy' word — the room is hot and airless, the lid is on tight. Most common in 闷热 (sultry, hot and stuffy), 闷头 (head-down, silently absorbed in something), 闷声 (quietly, without a word), 闷骚 (outwardly reserved but inwardly passionate). Rule of thumb: if the word is about temperature / airlessness / silence-on-the-outside, it's mēn; if it's about a sad or trapped feeling, it's mèn.