The etymology is designed to account for the meaning "universe", as consisting of elements composed in an orderly fashion and stable. From the formal point of view, it requires several manipulations, κεῖμαι supposedly accounting for κοσ- (which implies the loss of [s] and the change of the vowel) and ἅμα for -μος (ἅμα provides the [m]). Greek etymologists normally use ἅμα to account for the beginning of the word, as the first element of the assumed compound, not at the end of the word: but they had a theory according to which prefixes could sometimes be found at the end of the word, which is particularly clear with the privative prefix νη- (see σελήνη / σέλας + νη-), and presumably the reasoning was similar for ἅμα