na-adjective, noun
Describes something of enormous scale, size, or degree; often used in formal or literary contexts.
彼の功績は洪大なものだ。
His achievements are immense.
洪大な宇宙の広がりを思うと、自分の悩みが小さく感じられる。
When I think of the vast expanse of the universe, my own worries seem small.
The kanji 洪 means 'flood, vast', and 鴻 means 'large bird, great'. Both contribute to the meaning of immense scale. The exact historical derivation is uncertain, but the word is used in formal writing.