Green’s Dictionary of Slang

coinkidink n.

[deliberately ‘jokey’ mispron.]

(US campus) coincidence.

[US] P. Munro Sl. U.
[US]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.
[US]S. King Finders Keepers (2016) 298: Idn’t that an interesting co-inky-dink?