Green’s Dictionary of Slang

kybo n.

also kibo
[Khyber (Pass) n.]

(US) a privy.

Dahlskog Dict. 36: Kybo, n. Keep your bowels open, an expression used in children’s summer camps; hence, a john at a camp or campground; an outhouse.
[US]T. Berger Sneaky People (1980) 92: What happened to Horse? He flush himself down the kibo?