Green’s Dictionary of Slang

to confess, to betray one’s confederates.

1727		Swift ‘Clever Tom Clinch going to be hanged’ in Miscellanies V (1736) 145: The Hangman for pardon lay down on his Knee; Tom gave him a Kick in the Guts for his Fee, Then said, I must speak to the People a little, But I’ll see you all damn’d before I will whittle.
1847		Westmorland Gaz. 21 Aug. 2/1: A Yankee visitor declares that not even in his own country is there a cooler case of whittling on record than [...] at Marylebone where a gentleman, finding a respectable constituency vacant, walked into it, chiselled all he found there, and then cut his stick.
1870		Perryburg Jrnl (OH) 6 May 4/1: A New Englander [...] when he gets the best of as man he makes him ‘whittle down’.
1873		Sl. Dict.
1895		Crowe Aus. Sl. Dict. 95: Whittle, to turn informer.