To take any one napping; i.e. to come upon him unexpectedly, to find him asleep: as, He caught him napping, as Morse caught his mare.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
To Nap * NapperNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes