noun
writer's callus; writer's bump
A callus that forms on the finger from prolonged writing or drawing pressure.
長年ペンを持っていると、中指にペンだこができる。
If you hold a pen for many years, you develop a writer's callus on your middle finger.
彼はペンだこが痛くて、しばらく字を書くのをやめた。
His writer's bump hurt, so he stopped writing for a while.
たこ is the general word for callus or corn anywhere on the body; ペンだこ specifically refers to a callus caused by writing.
Compound of ペン (pen) and たこ (callus). The kanji form 胼胝 is a rare ateji for たこ.