Green’s Dictionary of Slang

1. to leave a restaurant without paying one’s bill; thus the mosh n., the practice of committing this fraud.

1857		‘Ducange Anglicus’ Vulgar Tongue 22: ‘Doing the mosh on the quiet.’ Dining at an eating-house, and not paying.
1859		Matsell Vocabulum.
1890–1904		Farmer & Henley Sl. and Its Analogues.

2. to pawn.

1923		J. Manchon Le Slang.
1930		(con. 1910–20s) D. Mackenzie Hell’s Kitchen 119: Mosh ... to pawn.