pack on

Verb

 * 1)  To add (fat, muscle, etc.) so as to increase mass or bulk.
 * 2) * 2006, Joel Marion, Men's Fitness, Dec/Jan, p. 102:
 * Most strength coaches agree that to gain an inch on your arms, you have to pack on at least 10 pounds of body mass.

Usage notes

 * Used especially in the contexts of health and bodybuilding.