Green’s Dictionary of Slang

1. a beggar who visits shops and claims to be collecting halfpence engraved with a harp, offering the shopkeeper 13 pence for a shilling’s worth and persuading them to empty all their coppers on the counter. While they search the pile, the palmer hides as many coins as possible.

1842		Edinburgh Rev. July 486: Palmers [...] visit shops under pretence of collecting harp halfpence; and to induce shopkeepers to search for them, they offer thirteen-pence for a shilling’s worth, when many person are silly enough to empty a large quantity of copper on their counters to search for the halfpence wanted. The palmer is sure to have his hand among it; and while he pretends to search for the harps, he contrives to conceal as many as possible in the palm of his hand [etc.].
1851		Lancaster Gaz. (Lancs) 28 June 3/5: Palmers are persons who visit shops under pretence of collecting harp halfpence [...] while he pretends to search for harps, he contrives to conceal as much as possible in the palm of his hand.
1864, 1867, 1870		Hotten Sl. Dict.
1873		Sl. Dict.
1895		C. Crowe Aus. Sl. Dict. 55: Palmer, a swindler or thief who can conceal jewellery while he pretends to be looking for some article.

2. a shoplifter [also pun on SE palmer, an itinerant monk, bound by vows of eternal poverty].

1838		Paul Pry 30 Sept. 183/3: [H]e generally waits on a debt for a palmer [...] and then gets his poor tool, George, to tuck him up the next day under ‘orders from the plaintiff’.
1859		Matsell Vocabulum 63: palmer A thief that adroitly slips jewelry from the top of a show-case into his pocket.
1869		J. Greenwood Seven Curses of London 88: A thief who secretes goods in a shop while a confederate distracts the attention of the shopkeeper is – a palmer.
1870		Rochdale Obs. 24 Dec. 6/2: ‘Palmers’ are men generally well dressed [...] working by twos [...] One of them men engages the attention of the shop-keeper [...] whilst the other is ‘palming’ his articles [...] and stowing them away into his pockets.