"to wrinkle, make uneven," c. 1600, in rumpled, of uncertain origin, perhaps a variant of rimple "to wrinkle" (c. 1400), from Old English hrympel "wrinkle" (possibly influenced by Middle Dutch rumpelen), related to Old English hrimpan "to fold, wrinkle" (see ramp (v.)). Related: Rumpled; rumpling. As a noun from c. 1500, "a wrinkle, a fold."
Also compare Middle English runkle "become wrinkled" (late 14c.), runkel (n.) "a wrinkle" (early 14c.), probably from Old Norse hrukka.