Green’s Dictionary of Slang

(US) a pocket.

1884		Leicester Chron. 14 June 12/2: I‘ve just been turned out [...] for having a bit of snout in my keck.
1892		Manchester Courier 3 Dec. 14/5: And he counted (sotto voce) all the dollars in his keck.
1900		A.H. Lewis ‘Joe Dubuque’s Luck’ in Sandburrs 156: I won’t do a t’ing but make it a t’ousand dollars in d’ kecks of the ducks who’s doin that song.
1906		A.H. Lewis Confessions of a Detective 203: But the thimble! Have you got it in your keck?