On The Level Meaning

(idiomatic) Honest(ly), sincere(ly), straightforward(ly), fair(ly).

Example:   He looks like a used car salesman, but he's really on the level, so you can trust him.
1912 August, Christy Mathewson, “Play Clean Ball”, Boys' Life, page 7:
  While I believe that the American people and the players want the game to be on the level, there are two or three things which, it strikes me, are not up to a high standard of honor.