opaque (adj.)

early 15c., opake, "dark, shaded, unlit" (a sense now obsolete), from Latin opacus "shaded, in the shade, shady, dark, darkened, obscure," of unknown origin. Spelling influenced after c. 1650 by French opaque (c. 1500), from the Latin. Meaning "impervious to the rays of light" is from 1640s. Figurative sense of "obscure, hard to understand" is from 1761. Related: Opaquely; opaqueness.