The phrase "bulletproof" refers to something that is strong and resistant, like a shield, that can protect or withstand bullets or any type of danger. Full definition
Perfect start to the day with a cup of bullet proof coffee. (ibreatheimhungry.com)
On the police safety side, sources say the governor is expected to propose more funding for the purchase of bullet proof vests and police car windows. (nydailynews.com)