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hammer
hopeless swimmer; person who can't swim; someone who sinks like a stone
かなづちの使い方を子供に教えるのは意外と難しい。
Teaching a child how to use a hammer is surprisingly difficult.
水泳が苦手でかなづちだと自嘲している。
He jokes self-deprecatingly that he cannot swim and sinks like a stone.
工具箱からかなづちを取り出して棚を組み立てた。
I took a hammer from the toolbox and assembled the shelf.