Bulletproof means protected against bullets, like the specially reinforced vests that police officers and soldiers wear. These vests contain layers of strong synthetic fibers, woven so tightly that when a bullet hits them, the fibers catch it and spread out its force across the vest instead of letting it punch through. Bulletproof glass works similarly: it's made of thick layers of glass and plastic that can stop bullets while still letting people see through it. You'll find bulletproof glass protecting bank tellers, presidents riding in cars, and guards at secure facilities.

People also use bulletproof to describe plans or arguments that seem perfect and impossible to attack or criticize. When a lawyer prepares a bulletproof case, she's gathered such strong evidence and built such careful reasoning that she believes no one can poke holes in it. A student might create a bulletproof excuse for missing homework (though teachers usually see through these!).

The term suggests total protection or complete preparation. Nothing is truly bulletproof forever: determined people can eventually find weaknesses in any defense or argument. But when something is bulletproof, it means someone has thought through every possible problem and protected against it.