Green’s Dictionary of Slang

boast n.

In phrases

make the boast (v.)

(US Und.) to obtain a pardon.

1909		‘The Lang. of Crooks’ in Wash. Post 20 June 4/2: [paraphrasing J. Sullivan] Making the boast is the obtaining of a release from a Pardon Board.
1910		J. Sullivan ‘Criminal Sl.’ in Amer. Law Rev. LII (1918) 891: ‘Making the boast’ is getting by the pardon board and obtaining a pardon.
1927		‘Jargon of the Und.’ in DN V 454: Make the boast, To secure a pardon.