expression, noun-modifying (irregular)
bottomless; unfathomable; immeasurable
Prenominal expression describing something so deep or vast that its limits cannot be perceived. Often used figuratively for emotions, potential, or darkness.
An unfathomable sadness showed in his eyes.
Bottomless darkness stretched deep inside the cave.
From 底 (そこ, 'bottom') + 知れぬ (しれぬ), the classical negative form of 知れる (しれる, 'to be known'). Literally 'bottom not known'.