means Burning slowly without flame, with smoke and low heat — or, figuratively, an intense emotion or attraction held quietly beneath the surface.

from From Middle English 'smolderen,' to suffocate or smother, tied to the smoke itself — the same root family as 'smother.' The word originally lived in the world of choking, slow-burning fires and stifled air; the romantic, slow-burn sense of a 'smoldering' look is a much later figurative borrowing, where the idea of contained heat made the leap from hearth to gaze.