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1. To cause something, usually hair, to stand on end with water, oil, gel, or some other product. A noun or pronoun can be used between "slick" and "up." I wanted to slick my hair up into spikes for our school photos, but Mom wouldn't let me.
2. To cause something to become more sleek, attractive, stylish, or sophisticated in appearance. A noun or pronoun can be used between "slick" and "up." I hate the way studios try to slick up music these days with all these computer enhancements and polish. These sorts of movies are inherently goofy and lowbrow—why try to slick them up?
slick something up
to tidy up something or some place. I have to slick this house up a little. Please slick up this room before company gets here.