level noun ▶ on the level honest, truthful, trustworthy US,1872 Also used as an adverb.
“Are you on the level?” he whispered. “Sorry?” I retorted, a bit indignant. “I mean”–he smiled–“are you on the square?” Then I twigged. This was freemason talk. One of their codes. — Jake Arnott, HeKillsCoppers, p. 45, 2001