To Tighten
To Become Tight, To Be Toned
intransitive verb, godan verb
Because the hiragana ends with an う sound, you know this word is a verb. The kanji itself means tighten, so the verb vocab version is to tighten.
What can you do if you want something to tighten or to become tight? You need to hug your sheep (し) really close until...
The sheep tightens into a tiny little ball. Cute!
このシートベルトは自動的に締まってくれます。
This seat belt tightens on its own.
大西さんの体はよく締まっていますね〜。
Mr Onishi's body is well toned isn't it!
初めての管理職ということで、身の引き締まる思いをしています。
This is my first managerial position and I’m very tense.