hopeless swimmer; person who can't swim
Metaphorical use likening a poor swimmer to a sinking hammer; usually written in kana. Often used self-deprecatingly or humorously.
私はかなづちだから、海には入らない。
I can't swim at all, so I don't go into the ocean.
He drove the nail in with a hammer.
He's a hopeless swimmer, so he doesn't go to the sea.
He smashed the wall with a hammer.
The metaphorical sense is usually written in kana.
Rarely used kanji form; 鉄 means iron and 鎚 is an alternative character for hammer.