noun
Colloquial word for a bump or swelling, especially on the head or forehead. Often used for a goose egg from a knock.
See also: 瘤
転んでおでこにたんこぶができた。
I fell and got a bump on my forehead.
頭をぶつけて大きなたんこぶができちゃった。
I bumped my head and got a big lump.
瘤 is a more general and neutral term for a lump or bump, while たんこぶ is colloquial and typically refers to a bump from an injury.
The origin is uncertain; likely a colloquial formation from たん (possibly related to たんこ 'lump') and こぶ 'lump, bump'.