adverb
forever; eternally; for a long time
Literary or formal adverb meaning 'for a very long time' or 'eternally'. Often used in set phrases like 永々と続く (to continue forever).
この伝統は永々と受け継がれてきた。
This tradition has been passed down for a very long time.
彼の名は永々と語り継がれるだろう。
His name will be told for generations to come.
永久 is a noun/adverb meaning 'permanence' or 'forever', more common in modern Japanese than 永々.
Reduplication of 永 (long, eternal) with the iteration mark 々, forming an adverb meaning 'for a very long time'.