Green’s Dictionary of Slang

knuckleball v.

[baseball imagery, the knuckleball is a difficult pitch with an erratic, unpredictable motion]

(US teen) to act deceptively, to disguise one’s intentions.

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[US]Baltimore Sun (MD) Sun. Mag. 4 Dec. 9/1: ‘Problem with you [...] is that you’re playing it late. You’re cool like a moose. You hit a boo-boo when you let him know you had the Zorros. Learn to knuckleball’.