Elliptic etymology: the real etymon is κίω "to go", not κινέω, as appears from other sources, but since κίω is a rare word, it is replaced here by a semantic equivalent κινέω. Anyway, Greek etymologists used to derivative's κινέω from κίω (Herodian), so that it could seem to be one and the same etymology. This is a descriptive etymology relying to the characteristic feature of the shadow that it is "attached" to an object or person. The [s] is accounted for either as an added letter (Theognostus, see σκιά / κίω) or, as here, as a syncopated form of the comitative prefix συν-. Notice that the etymology, starting from a verb of motion, defines σκιά as the shadow of animate beings since it "moves with": we must assume that it was thought to apply to the shadow of inanimate objects by extension.