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1. To load someone or something onto something, especially a vehicle. They really packed us on the tour bus. I didn't even have enough room to stretch my arms! We packed on as much as we could fit inside the van.
2. To gain fat or muscle mass, especially very quickly. I tend to pack on a few extra pounds during the holidays, so I make a point of hitting the gym in January to trim back down again. You can lift weights all you want, but you need to alter your diet if you want to really pack some muscle on.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.