Opaque (adjective)

1. Not able to be seen through; not transparent.
2. Not easily understood; not clear.

Origin:

Mid 16th century (in the sense "not transparent"): from latin opacus, from greek opakos "dark, shady", from opos "juice, sap".

Examples:

1. The windows were covered with opaque curtains.
2. The liquid in the bottle was opaque.
3. The instructions were opaque and difficult to follow.
4. The language used in the report was opaque to most people.
5. The company's accounting practices were opaque and hard to follow.