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To shake or otherwise move something soft or plush so that it becomes (or at least looks) fluffier. A noun or pronoun can be used between "fluff" and "up." I fluffed up the pillows on the couch before our guests got here.
fluff something up
to make something soft appear fuller or higher. Fluff your pillow up before you go to bed. She fluffed up her pillow before retiring.
To shake or ruffle something, especially something with feathers or fur, to make it softer, puffier, or larger: When my canary fluffs up its feathers, it looks round and fat. I fluffed up the cat's fur with a hairbrush. The pillows will be more comfortable if you let me fluff them up.