Green’s Dictionary of Slang

a fool, an eccentric.

1886		W. Newton Secrets of Tramp Life Revealed 9: Glock ... Half-wit.
1891		Birmingham Dly Post 31 Mar. 3/4: The ‘shallow blokes,’ [...] never recognised in the jolly, red-faced old gentleman [...] the strange little ‘glock’ (man) who had played ‘crib’ with them on the previous night.
2024		J. Meades Empty Wigs (t/s) 172: A mob of taproom oggies and glocks threw sticks and rotten fruit .