Refers to thickened, hardened skin from repeated friction or pressure, such as on hands or feet. In everyday use, たこ is the standard term.
I got a callus on my finger from playing the guitar too much.
After walking in new shoes, I got a corn on my heel and it hurts.
writer's callus; writer's bump
Usually written in kana; the kanji forms 胼胝 and 胝 are rare.
Katakana spelling is sometimes used for emphasis or in informal writing.
Rare kanji form; mainly encountered in medical or literary contexts.
Rarely used kanji form.
The origin is uncertain; it may be related to the word for octopus (蛸/たこ) due to a perceived resemblance, but this is speculative.