In centuries past, a 'cow's thumb' was a Scottish folk saying refering to ridiculously small measurement -- a thumb's width, but smaller, much smaller. While it is unclear if this usage ever became common in the rest of the UK, it was incorporated into a longer phrase was at least briefly popular.
'Done to a cow's thumb' was a fairly well known English idiom in the British Isles in the 1700s, with its first appearance in writing in 16711 and largely fading out of use before the 1800s, although we can only see those instances that made it into print.
Done to a cow's thumb means all in, finished, or that something is perfect. The most commonly referenced definition comes from Grose's Classical Dictionary of the Vulgar Tongue2, reading simply "done exactly". It is also worth noting that the original 1671 reference to "to a cow's thumb" was a mocking mondegreen of the French phrase "à la coutume", meaning "how it is done". It is possible, although perhaps not likely, that "cow's thumb" is nothing but a folk etymology trying to make sense of those damned frenchies.
1. Etymologicon Linguae Anglicanae by Stephen Skinner, 1671
2. Grose's Classical Dictionary of the Vulgar Tongue, by Francis Grose and Pierce Egan, 1823