resistant

Being resistant means to be immune or impervious to something. A vest that's resistant to bullets is bullet-proof.

Anything resistant has a certain power — specifically, the power to resist or fight off — something else. This word is often used medically, like when someone is resistant to a virus: that means they can't get the virus. You can also be resistant to someone's jokes, meaning they can't make you laugh. A raincoat should be resistant to water. If it's not resistant, that's a pretty bad raincoat. All resistant things are like a shield that something can't get through.