early 15c., from M.L. transparentem (nom. transparens), prp. of transparere "show light through," from L. trans- "through" (see trans-) + parere "come in sight, appear." Figurative sense of "easily seen through" is first attested 1590s. The attempt to back-form a verb transpare (c.1600) died with the 17c.
early 15c., opake, from L. opacus "shaded, shady, dark," of unknown origin. Spelling influenced after c.1650 by Fr. opaque(c.1500), from the Latin.