An expression of surprise, amazement, pleasure, unexpectedness; or, agreement with sentiments thereof

A thing (ting) that is mad. Derived from Jamaican.

When someone does something really bad in a funny way

In America, we'd say something is "crazy" if it's unbelievable or surprising (or indeed crazy). In London, the equivalent word is "mad". As in, "He's gone mad!" or "That's absolutely mad, man!"

A mad ting is a mad ting