A sat-nav is a little computer in a car that tells you which way to go. It talks to satellites in the sky, works out where you are, and then speaks or shows the next turn. The name is short for satellite navigation, and it is especially British; people say it the way they say “fridge” for refrigerator — everyday, not fancy. Though actually, a lot of drivers now just use a phone on the dashboard, and they still call that their sat-nav if they are British. In American English the usual word is GPS, even when the box is doing the same job. People also stretch it: “You’re my sat-nav” can mean “you keep telling me where to turn,” as a joke, not a real machine.