"spotted, marked with roundish spots of different colors or shades," early 15c., probably from a Scandinavian source akin to Old Norse depill "spot," Norwegian dape "puddle, splash of water." Or perhaps a back-formation from, or merger with, Middle English adjective dapple-gray "apple-gray" (late 14c.), used first of a horse, based on resemblance to the markings on an apple (compare Old Norse apalgrar "dapple-gray").
Dapple (adj.) "marked with spots, having spots of different colors or shades" (1550s), dapple (n.) "a spot, one of a number of various spots" (1570s), and dapple (v.) "to mark with various roundish spots of different colors or shades" (1590s) seem too late to be the source, but the relationship of all of them is uncertain.
Also, for origin of the sense, compare Middle English shimed (mid-15c.), of a horse, "dappled, dapple-gray," etymologically "shadowed," related to Old English scima "shade, glimmer."