This word popped up in a book I happened to be reading the other day, Appleby’s End, one of the more skittish and fanciful works of the late Michael Innes (the pen name of the Oxford scholar J I M Stewart). Inspector Appleby is investigating strange goings-on in a rural neighbourhood and visits an old woman, of whom the local vicar says, “Since girlhood she has been celebrated in this part of the countryside for her skill in vaccimulgence.” Putting it another way, she was a milkmaid.
Mr Innes was not the first to employ this weird word for the milking of cows, for it turns up in a whimsical letter written by Samuel Taylor Coleridge in November 1796: “Will you try to look out for a fit servant for us, simple of heart, physiognomically handsome, and scientific in vaccimulgence. That last word is a new one, but soft in sound, and full of expression. Vaccimulgence! I am pleased with the word.” Alas, few others have been, to judge from its limited appearances in print.
It is, as you may guess, derived from Latin vacca, a cow (which is also the origin of vaccine, because the first was derived by Dr Jenner from cowpox to guard against the much more serious smallpox). The ending is from the Latin verb emulgere, to milk out, which — as well as being the ultimate origin of emulsion — is the root of another very rare word, emulgence, the action of milking out, as for example in extracting money from the unwilling.