Green’s Dictionary of Slang

level adj.

(orig. US) honest, trustworthy, true.

1870		Orchestra 12 Aug. 331/1: To tell a woman her head is level is apparently a compliment in America [DA].
1902		E. Townsend Chimmie Fadden and Mr Paul 98: Get level, Duchess, or he ’ll run me in.
1919		H.C. Witwer Smile A Minute 355: I suppose that’s level.
1923		H.C. Witwer Fighting Blood 19: Is that level – you mean that, Judy?
1930		E. Glyn Flirt & Flapper 28: Flapper: Decent means level — on the square.
1943		R. Chandler Lady in the Lake (1952) 177: You’re a pretty level guy, Marlowe.
1962		Ragen & Finston World’s Toughest Prison 807: level – Can be trusted. ‘One who is “on the level”.’.
2006		G. Malkani Londonstani (2007) 109: I’m really admiring that desi today, keepin his shit level.

In phrases

(on) level chalks (adj.)

on equal terms.

1849

Morn. Chron. (London) 27 Nov. 5/2: Their rule, they, is to be ‘level chalks’ with their customers, be they what they may.