蒸
ジョウ, セイ, む.す, む.れる, む.らす

steam, heat, sultry, foment, get musty

JLPT2 G6 S13 F1552

烝# rise + 艸 grass/plant → *steam* rising from (cooked) vegetables → *sweaty*; *stuffy*.

[Full Kanji Etymology List]