Thames butter, n. — Green’s Dictionary of Slang

Thames butter n.

[the ‘South London Press …published a paragraph to the effect that a Frenchman was making butter out of Thames mud at Battersea. In truth this chemist was extracting yellow grease from Thames mud-worms’ (Ware)]

totally rancid butter.

1870		Liverpool Dly Post 24 Mar. 4/5: The Thames Mud Butter — Lately a sample of the grease from the thames mud at Battersea has been examined.
1871		Birmingham Dly Gaz. 23 Jan. 7/3: On the Thames butter question [...] when the liquid has been throughly cooled, there was a layer of pure butter at the top, in the centre [...] a layer of hair, dust, and cotton fibre, and at the bottom a kind of milky water.
1894		G.A. Sala London up to Date 358: An oleagineous stuff known as ‘Thames butter,’ was manufactured out of the slime and ooze of the filthy river and vended as genuine butter.
1909		J. Ware Passing Eng. of the Victorian Era.