to fasten a seatbelt in a car or on a plane
bugger off 
INFORMAL 
If you tell someone to bugger off, you're telling them to go away in a very impolite and aggressive way.
build on (1)
to add to what you've already succeeded in doing
build on (2)
to add a new section to a house or a building
build up (1)
to work at something and make it get stronger or bigger
build up (2)
to gradually increase
Bump into
to meet somebody by chance
Burn down
If something like a building or a forest burns down, it's completely destroyed in a fire.
burn out (1)
If a fire burns out, it slowly dies down until it stops burning altogether.
burn out (2)
to work too hard and suffer from physical and mental exhaustion
Butt in
to start talking when somebody else is already talking
Butt out INFORMAL
If you want to tell someone quite forcefully to mind their own business, you can tell them to butt out.
Butter up
to say nice things to someone before asking them to do something for you or to give you something
