All 4 Uses

opaque
in
The Story of Edgar Sawtelle
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But when it came to knowing his thoughts, Edgar could be as opaque as a rock.
p. 295.5 *

opaque = difficult to understand (see into)
IF EDGAR HAD BEEN INWARD and opaque to her at five, now he was a total mystery.
p. 298.2

opaque = difficult to understand
The once-narrow ribbon of smoke had become an opaque mass that belched from the top half of the barn's entryway.
p. 544.1 *

opaque = unable to be seen through
That had quickly lost its thrill, but perhaps his obsession with telling time had just been internalized, gone opaque.†
p. 296.2

Definitions:

(1) (opaque as in: opaque shower door)
not able to see through
(2) (opaque as in: opaque explanation)
difficult or impossible to understand -- especially due to complexity or obscurity
(3) (meaning too rare to warrant focus)