adverb
Dated, somewhat literary or poetic adverb expressing strong affirmation. In modern Japanese, じつに or 本当に are more common.
げに、その言葉は真実であった。
Indeed, those words were true.
げに美しき花よ。
Truly, a beautiful flower.
本当に is the everyday word for 'really'; げに is much more formal and old-fashioned.
Derived from classical Japanese げに, a combination of げ (real) and に (adverbial particle). The exact historical development is uncertain.