Comes from the Dutch word koolsla, kool meaning "cabbage" and sla meanng "salad". Although cabbage is the classic base, the term has come to refer to any shredded salad served cold in either a creamy dressing or acidic marinade.
Slaws marinaded in acidic pickles or brines go back to the Roman tradition of soaking raw vegetables in salt and vinegar until they wilted. The word salad comes from the Latin salata or "salted herbs". Slaws using mayonnaise didn't come along until much later since mayonnaise arrived on the scene in the 18th century.
Thanks to Good Eats by Alton Brown for this information.