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1. To become very plump, chubby, or rotund. Soak the dehydrated mushrooms in a cup of water for an hour, until they have plumped up and the water is nearly gone. I began plumping up almost as soon as I became pregnant.
2. To make someone or something plump, chubby, rotund, or fuller in shape. In this usage, a noun or pronoun can be used between "plump" and "up." My kids always come home for the holidays looking so skinny, so I always try to plump them up a bit. It has come to light that one of the biggest poultry producers in the country has been plumping up chicken meat with water to artificially increase their weight and subsequent price.
plump something up
to pat or shake something like a pillow into a fuller shape. Todd plumped his pillow up and finished making the bed. He plumped up his pillow.
1. To become more plump or chubby: I went off my diet and immediately plumped up.
2. To make something rounded or full in form: We plumped up the dried cherries by soaking them in water. The nurse brought me a pillow and plumped it up before putting it behind my head.