To fasten or secure oneself with a seatbelt or similar restraining mechanism.

To admonish someone to be careful or to take care.

ध्यान रखना

English Usage: You better belt up if you want to stay safe on the road.

Hindi Usage: अगर आप सड़क पर सुरक्षित रहना चाहते हैं तो आपको ध्यान रखना होगा।