Green’s Dictionary of Slang

stun! excl.

also stunner!

(UK juv.) an excl. used to underline the amusement one derives from another’s misfortune.

OnLine Dict. of Playground Sl. 🌐 stun, stunner n. Basically a word used to laugh at other peoples misfortunes, e.g. ‘My dog’s just died’ – hahah Stun! ‘I just got a case of crabs off Shirley Jones’ – ‘stunner!’.