To soil or begrime something. A noun or pronoun can be used between "dirty" and "up." I'm not dressing the kids for church until the last possible second so that they won't have a chance to dirty up their clothes.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
dirty something up
Rur. to get something dirty. Those pants are brand-new! Don't dirty them up! Don't dirty up your brand-new pants!
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.