Green’s Dictionary of Slang

to take someone by surprise, to catch someone off their guard.

1587		Greene Tritameron Pt II L iiii: The gentleman [...] taking her by the hand, made promise to performe the deeds [...] and with that Panthia, and the rest tooke them napping.
1593		Shakespeare Taming of the Shrew IV ii: Nay, I have ta’en you napping, gentle love.
1606		Return from Parnassus Pt II IV ii: Now may it please thy generous dignity To take this vermin napping, as he lyes In the true lappe of liberality.
1611		L. Barry Ram-Alley IV i: Do but conceale your selfe, and we shall catch My Sergeant napping.
c.1620		Fletcher Women Pleased III iv: And once I would have sworn I had taken her napping.
1628		R. Speed Counter-Rat F4: [A long-tail’d Rat] It was a petty-fogging Varlet, Whose back wore freeze, but bum no scarlet, And was tane napping with his Harlot, At Noddy.
1631		J. Shirley School of Complement III ii: He tooke me napping on Midsummer Eue.
1658		Mennis & Smith ‘Ballet of Shepheard Tom’ Wit Restor’d (1817) 304: Say on a tree she may see her Tom rid from all care, Where she may take him napping as Mosse took his Mare.
1663		S. Butler Hudibras Pt I canto 3 line 821: I took thee napping unprepared.
1675		C. Cotton Scoffer Scoff’d (1765) 182: I ne’er can think to take him napping.
1722		S. Centlivre Artifice Act II: Odd, there’s a pretty Penny to be made of these Cuckold-making Dogs, if one could but catch ’em napping.
1732		Ladies Delight 5: But Misan’s taken this just napping.
1762		Smollett Sir Launcelot Greaves II 203: I’d cheat my own vather, as the saying is—a must be a good hand at trapping, that catches the starns a napping.
1772		Bridges Burlesque Homer (3rd edn) 436: If there was a chance of snapping / A proper time to catch you napping.
1785		Grose Classical Dict. of the Vulgar Tongue n.p.: To take any one napping; i.e. to come upon him unexpectedly, to find him asleep.
1797		Bridges Burlesque Homer (4th edn) II 257: [as cit. 1772].
1811		Lex. Balatronicum [as cit. 1785].
1818		‘A. Burton’ Adventures of Johnny Newcome IV 200: Thus the Pilferers might wake him, If they should napping try to take him.
1821		R. Waln Hermit in America on Visit to Phila. 2nd series 25: I caught you napping.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue [as cit. 1785].
1842		Leeds intelligencer 10 Dec. 8/1: If they do raise that sum for the carrying on of their mischeivous projects, then shall we say that they have caught John Bull napping.
1848		J.R. Lowell Biglow Papers (1880) 69: One way or t’other hendiest is / To ketch the people nappin’.
1855		W.G. Simms Forayers 428: They’re cotch’d napping, on their haunches.
1865		H.L. Williams Joaquin 12: An idea which they had had of catching a detachment of Uncle Sam’s ‘blue-bellies’ napping.
1873		W.H. Thomes Slaver’s Adventures 130: Pull strong and silently, and we shall take them napping.
1885		Bulletin (Sydney) 24 Oct. 6/3: [A] capital photo of the ‘Vagabond’ in this interesting and dignified attitude was soon obtained. When Julian awoke and found how he had been ‘caught napping’ he swore, like Sala, in several languages.
1889		B. Mitford Fire Trumpet I 148: We don’t often catch Jack napping.
1897		Exeter Flying Post 26 Jan. 3/3: Did someone suggest a game of nap? Ah bet you don’t catch this cove napping. I pass.