wet ’un, n.² — Green’s Dictionary of Slang

wet ’un n.²

[wet ’un presumably means ‘wet brain’ and the disease must have been the same as, or at least similar to, the late-20C+ BSE, ‘mad cow disease’]

a diseased cow, technically unfit for human consumption, but often sold for conversion into sausages.

1864, 1867, 1870		Hotten Sl. Dict.
1895		Crowe Aus. Sl. Dict. 94: Wet ‘Uns, diseased cattle killed and made into sausages.