c. 1300 (implied in smoldering), transitive, "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 recorded by 1520s, fell from use 17c. (though smoldering persisted in poetry) and was revived 19c. The figurative sense of "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."