braggadocio, n. — Green’s Dictionary of Slang

braggadocio n.

[SE braggadocio, an empty, idle boast or boaster; thus the professional thief’s boast that they will never be caught]

1. a sentence of three months’ imprisonment given to a known thief or regular offender.

1860		Hotten Dict. of Modern Sl. etc. (2nd edn).
1864, 1867, 1870		Hotten Sl. Dict.