Syllable division derived from spelling — this word is not in the pronouncing dictionary, so no phonetic transcription is shown rather than a guessed one.
muscle-builder is a noun meaning "someone who does special exercises to develop a brawny musculature". It carries one recorded sense.
someone who does special exercises to develop a brawny musculature
Definitions, synonyms and semantic relations for muscle-builder come from Princeton University’s WordNet. Commonness is measured against a corpus of everyday English. Full sources and licences.