not transparent; not capable of being seen through or into
You can see to the bottom of a small lake, but the waters of the ocean are opaque.
opacus (shaded, in the shade, shady, dark, darkened, obscure) (Latin)
nebulous, opaque, turbid
OP (opposite) and AQUE (aqua)-- opposite of water -- means not transparent. The OPening wAs QUEstionable because no light was coming through. Therefore, it was OPAQUE.