smoke, soot, tobacco, cigarettes
菸 centers on the products and byproducts of burning plant matter: smoke, soot, tobacco, and cigarettes. The meanings are linked by the idea of plant material turning into airborne particles.
菸 combines 艹 (grass/plant) with 於, which likely contributes the sound. The character is associated with smoke and tobacco, but the exact historical development is uncertain.
Plants 艹 under a covering 於 produce smoke when burned. Picture a pile of tobacco leaves smoldering and releasing thick smoke and soot.
For エン, imagine the smoke rising from a burning yen note made of tobacco leaves: yen -> エン, and the smoke curls upward.
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