We don't have synonyms for bulk up yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are buck up, bulks up and bunk up.
(v) :
(bodybuilding) To train the body with a high-calorie diet and intense weightlifting in order to increase the overall mass of the body, especially the muscles.
(v) :
(bodybuilding) To train the body with a high-calorie diet and intense weightlifting in order to increase the overall mass of the body, especially the muscles.