Green’s Dictionary of Slang

(US) dubious, untrustworthy.

1915		S. Ford Shorty McCabe on the Job 263: With things gettin’ that gummy, I was beginnin’ to feel like a cold boiled potato served accidental with the pie.
1925		S. Lewis Arrowsmith 170: It was a Ford, five years old, with torn upholstery, a gummy motor.
1927		D. Hammett ‘The Big Knockover’ Story Omnibus (1966) 316: This game I was playing in was — it had to be — gummy around the edges.
1947		(con. 1944) J.H. Burns Gallery (1948) 71: In the gummy city of Oran [...] He would wake in the mornings with a feeling of the most intense displacement.