taru-adjective, adverb taking the 'to' particle
Obsolete literary term describing something vague, hazy, or indistinct, often used in classical or poetic contexts.
In an old waka poem, one finds the expression 'the dim light of the moon'.
彼の記憶は曖曖として、はっきりしなかった。
His memory was dim and unclear.
The historical derivation is uncertain; the kanji 曖 is associated with dimness or obscurity, and the reduplicated form 曖曖 intensifies that meaning.