A massive hard sweet, usually a big round ball, that you suck for ages while it slowly changes colour and ruins your teeth. Classic corner shop treasure. By extension, it’s anything that keeps someone’s mouth busy or shuts them up for a bit, especially a kid who’s chatting absolute nonsense.
"He wouldn’t stop yapping on the bus, so Gran lobbed him a gobstopper from her handbag. Silence. Ten minutes later he’s back like, Mum, can I have another?"