hurrah!; long life; congratulations; full vent
Archaic or highly literary celebratory exclamation, often used in old texts or set phrases like 万歳. The gloss 'full vent' suggests an extended meaning of giving full expression to something, but this is rare.
In old plays, there are scenes where they shout 'banjō, banjō!'
The expression 'banjō no kien' means one's spirits are at their highest.
stormy and full of drama; full of ups and downs
in high spirits; talking big; full of enthusiasm
rising cloud of yellow dust
From 万 (ten thousand, myriad) and 丈 (length, measure), literally 'ten thousand jō', suggesting something immeasurably great or long. The celebratory use is likely an extension of this idea of boundlessness.