"salted and peppered raw egg, drunk in booze or vinegar," by 1878, American English, from prairie + oyster (in reference to the taste or the method of consuming it). Also called prairie-cocktail (1889). Prairie-oyster as "fried calf testicle," considered a delicacy, is by 1941.
PRAIRIE OYSTER. This simple but very nutritious drink may be taken by any person of the most delicate digestion, and has become one of the most popular delicacies since its introduction by me at Messrs. Spiers and Pond's. Its mode of preparation is very simple. Into a wine glass pat a new-laid egg ; add half a tea-spoonful of vinegar, dropping it gently down on the inside of the glass ; then drop on the yolk a little common salt, sufficient not to quite cover half the size of a threepenny-piece; pepper according to taste, The way to take this should be by placing the glass with the vinegar furthest from the mouth and swallow the contents. The vinegar being the last gives it more of an oyster-like flavour. [Leo Engel, "American & Other Drinks," London, 1878]