c. 1300 (implied in smoldering), "to smother, suffocate," related to Middle Dutch smolen, Low German smelen, Flemish smoel "hot," from Proto-Germanic *smel-, *smul-. The intransitive meaning "burn and smoke without flame" is first recorded 1520s, fell from use 17c. (though smoldering persisted in poetry) and was revived 19c. Figurative sense "exist in a suppressed state; burn inwardly" is from 1810. Related: Smouldered; smolderingly. Middle English also had a noun smolder meaning "smoky vapor, a stifling smoke."
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Harper, D. (n.d.). Etymology of smoulder. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/smoulder