means Not letting light pass through, so you can't see what's behind it — and, by extension, hard to understand or see into.

from From Latin 'opacus,' meaning shaded, shadowy, or dark — the kind of word you'd use for a tree's deep shade. It came into English through French, and early on it actually got spelled 'opake' for a while, before the Latin spelling muscled its way back in. The figurative sense of 'unclear, impenetrable to the mind' is a later stretch from the literal blocking of light.