pitch-darkness; total darkness
Emphatic expression for complete, impenetrable darkness, often used in literary or dramatic contexts.
洞窟の中は真の闇だった。
Inside the cave it was pitch-dark.
Compound of 真 (しん, 'true') + の (possessive particle) + 闇 (やみ, 'darkness'). The exact historical derivation is uncertain, but the phrase is used to express absolute darkness.