Green’s Dictionary of Slang

a small person, a dwarf; also used as excl. (see cite 1663).

1570		Marriage of Wit and Science V v: But what, no force ye are but Jack Sprot to mee.
1648		Mercurius Anti-Mercurius n.p.: Every Jack-sprat that hath but a pen in his ink-horn is ready to gather up the Excrements of the Kingdom.
1652		J. Hilton Catch that catch can 10: [W]ouldst thou laugh and be fat, there's not any like to that, to make Jack Sprat a man of Kidney.
1663		T. Porter The female victor n.p.: 2 Drawer. These will spoile thy drinking. Cellerman. My drinking Jack Sprat, I could box thee sirrah.
1670		J. Ray Proverbs 211: Jack Sprat he loved no fat.
c.1698		B.E. Dict. Canting Crew n.p.: Jack-sprat, a Dwarf, or very little fellow, a Hop-on-my-thumb.
1725		New Canting Dict. [as cit. c.1698].
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. c.1698].
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
1811		Lex. Balatronicum.
1818		‘Thomas Brown’ Fudge Family in Paris Letter III 28: There never was seen such a race of Jack Sprats.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1839		Manchester Courier 29 June 2/3: The little knowing-looking cockney orator, who they call Jack Sprat.
1859		Matsell Vocabulum.
1862		E. de la Bédollière Londres et les Anglais 315/2: jack sprat, petit homme.
1873		Sl. Dict.
1881		Trumble Sl. Dict. (1890).