taru-adjective, adverb taking the 'to' particle
vast; boundless; calm and expansive
Literary adjective describing a vast, calm expanse, such as a great body of water or the open sky. Often used in classical or poetic contexts.
湖面は蕩々として、まるで鏡のようだった。
The lake surface was vast and calm, just like a mirror.
蕩々たる大海原を眺めていると、心が洗われる思いがした。
Gazing at the boundless expanse of the ocean, I felt my heart being cleansed.
The exact derivation is uncertain; the kanji 蕩 and 盪 both carry meanings of 'moving' or 'washing away,' and the reduplicated form likely emphasizes boundlessness and tranquility.