Looking at the cartoon @ttf posted the McD’s coffee was considered too hot at 180F, which is ‘only’ about 82C.
However the point was that the coffee soaked Stella Liebeck’s trousers, which were synthetic material it seems, and superheated them, resulting in her being exposed to that heat for a prolonged period.
If you spill boiling water on your skin (hand say) it does burn and it’s painful but it either runs off or you shake it off quickly.
So from the looks of this case, it’s got into her trousers or maybe her tights, resulting in a vicious burn.
Not sure what to say about it, mind you. Driving with hot drinks is definitely pretty dangerous.