蒸し暑い
むしあつい · mushiatsui · JLPT N3 · i-adj
humid
🥵💦 蒸し暑い = steaming (蒸す) + hot (暑い): mushy, steamy heat
蒸し comes from 蒸す, to steam, the verb used for steaming rice or buns. 暑い means hot weather. Put together, it is weather that steams you like a dumpling. Mushi also sounds like mushy, which fits that damp, heavy air.
蒸 — grass 艹 + 丞 (phonetic) + fire 灬
Originally depicted plant stalks (艹) being heated over fire (灬), with 丞 providing the sound. The character first meant rising steam from burning hemp stalks used as torches, then broadened to mean steaming food or vapor rising. Modern uses extend metaphorically to 'flourishing' or 'thriving.'