A general nick-name for a Scotchman, as Paddy is for an Irishman, or Taffy for a Welchman; Sawny or Sandy being the familiar abbreviation or diminution of Alexander, a very favourite name among the Scottish nation.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Saw * ScabNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes