A violent blow on the mouth. The milling cove tipped the cull a muzzler; the boxer gave the fellow a blow on the mouth.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Muzzle * MyntNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes