Smouldering or smoldering is the slow, flameless form of combustion, sustained by the heat evolved when oxygen directly attacks the surface of a condensed-phase fuel.

What does it mean when someone is smoldering?

If you're silently angry, your contained rage is smoldering, too. Just like a fire that doesn't go out completely, this word is used for emotions that also refuse to die out — they just keep going, though under the surface. If you keep a lid on your feelings, but are mad at someone, your feelings are smoldering.

What is smoldering good looks mean?

If you replace "smouldering" with "sexy", you are still saying roughly the same thing: Sheldon's smouldering good looks = Sheldon's sexy good looks.