Green’s Dictionary of Slang

one who steals children, esp. for use as servants or labourers on the plantations.

a.1674		‘The Thief-Ketcher’s Song’ Head Canting Academy (1674) 145: The eighteenth a Kid-napper, Spirits young men.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue n.p.: Kidnapper. Originally one who stole or decoyed children or apprentices from their parents or masters, to send them to the colonies; called also spiriting: but now used for all recruiting crimps for the king’s troops, or those of the East India company, and agents for indenting servants for the plantations.
1815		(con. 18C) W. Scott Guy Mannering (1999) 191: The devil can scarce save Dirk Hatteraick from being hanged for a murderer and kidnapper.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue [as cit. 1785].
1864, 1867, 1870		Hotten Sl. Dict. 165: KIDNAPPER, one who steals children or adults.