noun-modifying (irregular)
indescribable; inexpressible; exquisite
A literary, prenominal expression meaning something is so wonderful, beautiful, or profound that it cannot be put into words. Often used in poetic or emotional contexts.
See also: えもいわれぬ
えも言えぬ美しさに、ただ見とれるばかりだった。
I could only gaze in wonder at the indescribable beauty.
彼女の歌声には、えも言えぬ魅力がある。
Her singing voice has an inexpressible charm.
Very similar in meaning and usage; えも言えぬ is slightly more common and both are literary prenominal adjectives meaning 'indescribable'.
A more everyday, analytical way to say 'cannot express in words', while えも言えぬ is a fixed, emotive literary phrase.
From え (old potential form of 得る, 'can') + も (even) + 言えぬ (old negative of 言う, 'say'), literally 'cannot even say'. The exact historical development is uncertain, but it is a fossilized classical expression.