taru-adjective, adverb taking the 'to' particle
deep and dark; profoundly dark
A literary, somewhat archaic word describing a deep, quiet darkness, often with a sense of mystery or the supernatural. Used as a 'taru' adjective or adverb with と.
The depths of the forest were wrapped in a deep, dark gloom.
I walked through the underground passage that stretched on darkly and endlessly.
Alternate spelling without the iteration mark; less common but still encountered.
Reduplication of 幽 (ゆう), meaning 'deep, dark, secluded', forming a literary adjective/adverb. The exact historical derivation is uncertain.