It is an abbreviation of saying "Swear on the friendship no crosses count." The original abbreviation was "S F on the Ny's N C" - pronounced (S F on the enys N C). Over time that abbreviation became just simple S F, which than eventually became Bada Bee. You can never lie when someone asks "Bada Bee." It would be the ultimate act of deception and end ones friendship with that person immediately.
Chet: Yo I banged that girl over there last night.
Jeebs: Bada bee