foot warmer, (kokuji)
燵 is a Japanese kokuji that centers on a foot warmer, a device that uses fire to warm the feet.
燵 is a Japanese kokuji created by combining 火 (fire) with 達, which likely contributes the sound タツ. It was coined to represent a foot warmer, a concept not covered by existing Chinese characters.
Fire 火 on the left and 達 on the right: imagine a fire reaching (達) your feet to warm them, like a traditional foot warmer.
For タツ, picture a foot warmer shaped like a tats (a made-up word for a cozy heated mat): tats -> タツ, and the fire keeps your feet warm.
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kotatsu; table over an electric heater, with a hanging quilt to retain heat (orig. over a charcoal brazier in a floor well)
kotatsu not put away but used in spring
square latticed wooden frame for a kotatsu
(electric) kotatsu table
futon or cloth attached to the kotatsu's frame; cover for kotatsu
portable brazier