The word butler derives from the old French bouteillier, and identified the cup-bearer or the one in charge of the bottles in large households. Bottle and the French equivalent both come from the medieval Latin buticula, a diminutive of butties, a cask, which is also the origin of the English word “butt”, given to large wooden container for liquid.
The beer cellar in medieval times would have contained butts or wooden casks, not glass bottles. So the buttery originally had nothing to do with butter but was the place for storing the butts.
Only later was the word extended to mean somewhere that provisions in general were stored, perhaps because people mistakenly made that association.
Through a complicated process that had to do with the loss of gentlemen servants and changes in social organization, the butler slowly rose to be in charge not only of the buttery, but also of the ewery (where the napkins and basins for washing and shaving were kept) and the pantry (where the bread, butter, cheese and other basic provisions were stored), and later still he took over the cellerer’s duties of looking after the wine.
This eventually became one of his principal duties. By the middle of the nineteenth century, the role of the butler reached its full flowering as head of the male domestic servants, in larger households sometimes the butler was given a whole suite of rooms dedicated to his various functions. In the early twentieth century, social change meant the butler almost vanished as a breed.
Toward the end of the twentieth century, the butler has been reinvented as a kind of Swiss-Army-knife, all-purpose household manager. Often the sole permanent servant,, the modern butler may be required to organize his master’s travel arrangements, supervise redecorating the house as well as serve the wine at formal dinners.
“Butler” is one of those words which has survived almost unchanged in the language for several hundred years, but whose meaning has progressively changed along with his duties. But as few of us encounter a real butler these days, even fewer than in his heyday, our understanding of the word is stuck in a fantasy world of Wodehousian invention or fictionalized film characters.