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1. To wash the inside of something very thoroughly and vigorously. A noun or pronoun can be used between "scrub" and "out." Did you scrub out the pots and pans like I asked? I've got to take this apart and scrub it out from the inside.
2. To clean some substance or residue out of the inside of something very thoroughly and vigorously. A noun or pronoun is used between "scrub" and "out"; often followed by "of (something)." You can't just leave the pot to soak overnight—you've got to scrub all the burnt food out with a scouring pad. I've got to scrub the gunk out of my keyboard, or it could end up permanently broken.
scrub something out
to clean out the inside of something by rubbing or brushing. Please scrub these pots out and put them away. Jim will scrub out the pots.