Green’s Dictionary of Slang

(US Und.) a jail sentence of one hour less than a whole year, thus avoiding the mandatory loss of citizenship that in some states comes with a one-year sentence.

1924		‘Digit’ Confessions of a Twentieth Century Hobo 11: Eleven, twenty-nine, twenty-three...A favourite sentence of some judges, meaning eleven months, twenty-nine days, twenty-three hours. In some states one loses one’s citizenship by a year in jail. [...] 161: Every man [...] had been caught, and they were now doing eleven, twenty-nine, twenty-three.
1925		G.H. Mullin Adventures of a Scholar Tramp 252: The State of Texas had an ‘eleven-twenty-nine law’.