Green’s Dictionary of Slang

(US Und.) a card-sharp who deals from the bottom of the pack.

1940		D. Maurer Big Con 183: All those starters are subway dealers.
1956		Dayton Dly News (OH) 16 Sept. 145/1: ‘He’s a subway dealer’ [...] a seldom used synonym for bottom dealer.
1963		News (Paterson, NJ) 3 July 42/2: What he didn’t know was that this particular cardster was a subway dealer. The ace of spades was on the bottom.
1974		L.A. Times Magazine 10 Feb. 33/2: If he’s a really superior ‘subway dealer,’ it is beautiful to behold.