Wiktionary
pack on
vb. (context transitive English) To add (fat, muscle, etc.) so as to increase mass or bulk.
WordNet
pack on
v. gain (weight); "He packed on two pounds over the summer"
Usage examples of "pack on".
A hunted buck from the south, many, many Rains ago, came hither from the south, not knowing the Jungle, a Pack on his trail.