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To cause hair lie flat on the head with water, oil, gel, or some other product. A noun or pronoun can be used between "slick" and "down." I woke up with this annoying cowlick, and now I have to slick my hair down with gel. I've always liked the look of boys who slick their hair down and part it on one side.
slick something down
to brush or comb down hair, usually with some sort of dressing or water. He used something gooey—grease or something—to slick his hair down. Please slick down your hair. You look a mess.