The head, probably as being the fort and citadel of a man: from SCONCE, an old name for a fort, derived from a Dutch word of the same signification; To build a sconce: a military term for bilking one’s quarters. To sconce or skonce; to impose a fine. ACADEMICAL PHRASE.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
See also the definition in Nathan Bailey's 1736 dictionary of canting and thieving slang.
School Butter * ScotNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes