smoulder

To smoulder is to burn slowly without flame. Heat and smoke, and no fire showing.

It is what a bonfire does overnight and what makes a discarded cigarette so dangerous, since something smouldering can burst into flame hours later. That slow hidden quality gives the figurative sense, and smouldering resentment is anger kept out of sight. There is a third and warmer use about looks, since a smouldering glance is an intensely attractive one, which has kept the word in fashion writing for a century. American English spells it smolder.