taru-adjective, adverb taking the 'to' particle
Literary, often used to describe an expansive sea, sky, or abstract boundlessness. Functions as a taru-adjective or adverb with と.
茫洋とした大海原を眺める。
Gaze at the vast expanse of the ocean.
彼の未来は茫洋としてつかみどころがない。
His future is boundless and elusive.
The exact derivation is uncertain; the kanji 茫 (boundless, vague) and 洋 (ocean) combine to suggest a vast, oceanic expanse.