buckle up

/B AH1 K AH0 L AH0 P/
verb
  1. 1

    To fasten with a buckle.

  2. 2

    To fasten one's seat belt or safety belt.

    โ€œBuckle up every time you drive somewhere in a car, and make sure your passengers buckle up, too.โ€

Synonyms

Source: https://en.wiktionary.org/wiki/buckle%20up