Green’s Dictionary of Slang

bong adj.

[Fr. bon, good]

(US) excellent.

[Aus]Queenslander (Brisbane) 27 Mar. 3/4: Things that gave pleasure were ‘tray bong’.
[UK]P. MacGill Moleskin Joe 99: R.C. parade cushy, tray bong, so I signed on as an R.C. [Ibid.] 183: The other beasts ain’t no bong.
[US]Wentworth & Flexner DAS.