Green’s Dictionary of Slang

ad lib n.

[SE ad lib, to extemporize; ult. Lat. ad libitum, as much as one desires]

(orig. US) a pointed, provocative or sarcastic comment.

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[US]G. Lee ‘Trouper Talk’ in AS I:1 36: ‘Can the ad lib!’ which means, politely, ‘Will you be good enough to hush!’.