Green’s Dictionary of Slang

(US) something small, esp. when neat, trim or dainty.

2000

C. Cook Robbers (2001) 44: You gotta have skinny ones [i.e. ankles], the guy said. Real dinkers. [Ibid.] 261: Bronson used a thirty-two dinker [i.e. a gun]. This guy carries a three-fifty-seven Sig Sauer.