Green’s Dictionary of Slang

(US) an idiot.

1889		‘Mark Twain’ Connecticut Yankee xxx 382: As a rule, a knight is a lummux, and sometimes even a labrick.
1894		‘Mark Twain’ Pudd’nhead Wilson 8: He’s a labrick — just a Simon-pure labrick, if ever there was one.
1915		M.G. Hayden ‘Terms Of Disparagement’ in DN IV:iii 204: laverick, a contemptuous term for a stranger. ‘Go to the door. Some laverick is out there.’.