Green’s Dictionary of Slang

(US campus) coincidence.

1927		J. Woods Brooklyn Times Union : And wouldn’t it be a funny co-inky dink if Sharkey and Mr. Up-and-Dump-’Em came to grips for the heavyweight title.
1989		P. Munro Sl. U.
1997		K. Kainulainen ‘University Euphemisms in Calif. Today’ 🌐 I will consider expressions surrounding something bad or discreet as euphemism and other invented expressions as slang. Therefore [...] ‘What a coinkidink!’ meaning ‘What a coincidence!’ [...] would be slang.
2015		S. King Finders Keepers (2016) 298: Idn’t that an interesting co-inky-dink?