noun
kotatsu bug; person who stays under a kotatsu all winter
Colloquial, humorous term for someone who spends the cold months curled up under a kotatsu and rarely moves. A pun on かたつむり (snail), likening the person to a bug or snail that lives in the kotatsu.
冬になると、彼は完全にこたつ虫になってしまう。
When winter comes, he turns into a total kotatsu bug.
休日はこたつ虫のように動かない。
On my days off, I don't move, just like a kotatsu bug.
A modern colloquial coinage blending こたつ (kotatsu) and 虫 (bug), with a pun on かたつむり (snail). The exact origin is uncertain, but it likely arose from humorous comparisons of kotatsu-loving people to bugs or snails.