Classic scam mastered by Frank Gallagher from Shameless. Involves jumping out in front of a moving car as your partner distracts them, then shaking them down for cash. Also the slang term for the surprise Trump got when several respected diplomats jumped out and testified against him regarding his extortion campaign in Ukraine.
Definition 1:

Frank: Bro, I'm short on cash. Let's do a Ukrainian peek-a-boo!
Frank's creepy friend: Nah, man, people don't carry cash anymore; it's not worth it.

Definition 2:

Rob: Dude, did you see what happened with Trump!?
Tom: Yeah man, he got a serious Ukrainian peek-a-boo!
by Notdevinnunes November 25, 2019
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