Green’s Dictionary of Slang

something or someone excellent; the best example.

1910		J.L. Waugh Cracks wi’ Robbie Doo (1914) 110: Weel dune, Robin Hood; dash it, man, but you’re a beeser .
1932		M. Marshall Travels of Tramp-Royal 85: I cooked a beezer of a breakfast.
1938		Arbroath Herald 30 Sept. 5/7: The reward we’ve got for a good job — anither order and crivans! she’s a beezer.
1997		Share Slanguage.