late 14c., from amidde (c.1200), from O.E. on middan "in the middle," from dative singular of midde "mid, middle" (see 'midde); the phrase evidently was felt as "in (the) middle" and thus followed by a gen. case, as we would follow it today with of. (See amidst for
further evolution along this line). The same applies to equivalents in Latin (in medio) and Greek (en meso), both originally adjective phrases which evolved to take the genitive case. But in later O.E. on middan also was treated as a prep. and followed by dative. Used in compounds from early 13c. (e.g. amidships, attested from 1690s and retaining the genitive, as the compounds usually did in early M.E., suggesting this one is considerably older than the written record of it.)