We don't have synonyms for pack on yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are pack in, tack on and hack on.
°To load the body with additional mass, in the form of fat or muscle.
"2006: Most strength coaches agree that to gain an inch on your arms, you have to pack on at least 10 pounds of body mass. — Joel Marion, Men's Fitness Dec/Jan, p.102."
°To load the body with additional mass, in the form of fat or muscle.
"2006: Most strength coaches agree that to gain an inch on your arms, you have to pack on at least 10 pounds of body mass. — Joel Marion, Men's Fitness Dec/Jan, p.102."