He’s a riot

Riot, in the sense of a major outbreak of violence, is nothing to laugh at. But the word riot has many other meanings. There is a “riot of color,” meaning an unlimited profusion of colors – as in the colorful displays put on by trees in Northern states when the leaves change with the coming of autumn. What’s more, the original riots were debauched revels – not necessarily at all violent – in which fun and games played a major role. So the expression He’s a riot! – while somewhat slangy – has a perfectly legitimate history.