Something used to wipe up semen after masturbation, or alternatively, masturbate into if the pervert in question is somewhere where he'd perfer to leave things as they were when he entered - such as a friend's house or that of his parents. Can be anything from age-old classic, the dirty sock, to a hand towel.
When selecting some random scrap of fabric for use as your jizz rag, do remember that semen can leave unsightly stains on black fabrics.
A jizz rag is simalar to a catch rag, being that it is used to wipe up or catch cum after jerkin the gherkin. The difference is that a jizz rag is not kept, but rather, thrown away after use. Common jizz rags are usually paper towels, toilet paper, love letters.