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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.