Could 'unify', 'attach', 'click', or 'fix' be a good synonym for 'fasten'?

[learning54]

Hi teachers,
Could 'unify', 'attach', 'click' or 'fix' be a good synonym for 'fasten' in the following sentence? Is there a better one?
Fasten your seat belt and start the car Robert.

Can I also say, 'Fasten your belt' around the waist? I mean, to keep the trousers from falling around the ankles.

Thanks in advance.

 

[Rover_KE]

The answer to your first two questions is no. You might hear 'buckle up' but it's not better than fasten, which is just about perfect.

Yes to the third question.

Rover

 

[emsr2d2]

I would almost always use "fasten" with seatbelt but I don't think I've ever used it for a belt around your waist. I would say "Do your belt up to stop your trousers falling down". I would not use either "fasten" or bother with "around your waist".