Green’s Dictionary of Slang

1. a swindler who specializes in defrauding pawnbrokers; also attrib.

1883		Dly Gaz. for Middlesborough 21 July 5/5: Within the limits of the metropolis there are at least a score of the ‘mosker’ tribe.
1883		Daily Tel. 9 July 3, col. 1: The mosker... is, in slang vernacular, one who makes a living by taking advantage of the business incapacity of persons engaged in the pawnbroking trade, and by subtle wiles and subterfuge imposes on their credulity and weak good nature .
1895		Mirror of Life 13 Apr. 14/1: [T]he famous, or infamous, Diamond Bob is dead. He was what we call in London a ‘mocker’ (pawner), and a big one too [...] he spent all [...] his life swindling pawnbrokers in the big cities of New York and Europe.
1931		Nott. Eve. Post 11 July 1/6: ‘I have found what is known as a “mosker” — a professional pawner,’ said a detective.
1946		S. Jackson Indiscreet Guide to Soho 121: He was a highly efficient ‘mosker,’ a crook who specialises in pawnbroking swindles.
1956		(con. 1900s) R.T. Hopkins Banker Tells All 126: What has become of those shadowy individuals who were known to the police as lil-fakers, moskers and duffers? [...] A mosker is a gentleman who endeavours to pledge articles of jewellery with pawnbrokers for considerably more than their actual worth.

2. a pawnbroker’s client, no criminality suggested.

1895

Mirror of Life 18 May 7/4: Another royal personage who has been compelled to apply to pawnbrokers for ready money is King Milan of Servia, who [...] pledged the jewelled sword which forms part of the regalia of the rulers of Servia, and is considered one of the most valuable heirlooms of the Obrenovitch family.