The use of 'sick' to mean impressive is now spreading and has become as

[World's End Stella]

I never understood the combination of tuna and melted cheese. Always seemed wrong to me.

However, I doubt this derivation. Surely it's as simple as just being someone who melts when things get too hot?

Dunno, Burney. He uses it because his mates in his house at school use it, I expect. Although that was the explanation he offered me.