Green’s Dictionary of Slang

a petty thief.

1508		Dunbar ‘A General Satyre’ in Mackenzie Poems (1932) 152: Sa mony theivis and mycharis weill kend.
1530		Palsgrave Lesclarcissement de la Langue Francoyse n.p.: Substances: Mecher, a lytell thefe; Micher, a lytell thefe.
1573		T. Tusser Five Hundred Pointes of Good Husbandrie (1878) 20: Once placed for profit [...] beware of such michers as thease.
1848		D.O. Madden Revelations of Ireland 116: May the devil fly away wid you, you micher from Munster.