A thief who picks locks, so called from the gilt or picklock key: many of them are so expert, that, from the lock of a church door to that of the smallest cabinet, they will find means to open it; these go into reputable public houses, where, pretending business, they contrive to get into private rooms, up stairs, where they open any bureaus or trunks they happen to find there.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
See also the definition in Nathan Bailey's 1736 dictionary of canting and thieving slang.
Gilly Gaupus * Gimblet-eyedNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes