Syllable division derived from spelling — this word is not in the pronouncing dictionary, so no phonetic transcription is shown rather than a guessed one.
unprompted is an adjective meaning "proceeding from natural feeling or impulse without external stimulus". It carries one recorded sense.
proceeding from natural feeling or impulse without external stimulus
“an impulsive gesture of affection”
Definitions, synonyms and semantic relations for unprompted come from Princeton University’s WordNet. Commonness is measured against a corpus of everyday English. Full sources and licences.