taru-adjective, adverb taking the 'to' particle
Obsolete literary term describing something vague, hazy, or indistinct, often used in classical or poetic contexts.
古い和歌に「曖曖たる月の光」という表現が見られる。
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.