netherthought

An internal or subconscious thought; a hidden, underlying, secretive, or furtive thought.

She had looked at him with sad intensity in the eyes, as if trying to fathom any nether thought that he might have.

[…]he had superficially forgotten that the readers' opinions of his story were in, while his nether thought writhed in anguish around the question of what their opinions were.

A low, base, or vile thought.

He has souped himself up with a couple of mighty croutons, this low-slung prince of the netherthought, and will bring furious ancient knowledge ringing around our inner ears.