A restaurant employee who orders and maintains the wines sold in the restaurant and usually has extensive knowledge about wine and food pairings (AHD).You'll sometimes see a sommelier defined as a wine steward, though technically there's a fair bit of expertise that's well beyond that of a steward.
Now, for the etymology. Sommelier is a word English borrowed from French; it's derived from Old French, from *sommerier, meaning "beast of burden," that is, a pack animal. Old French *sommerier was, it appears, borrowed into French via Provencal, and before that, from Vulgar Latin *saumrius.
The basic concept is that the sommlier, an expert on the storing and selection of wines, was a position that was once associated with cargo shipments via pack animals. The same Latin root that gave us sommlier also gave us the word summer, not the word for the season, but the summer that means
1. A heavy horizontal timber that serves as a supporting beam, especially for the floor above.Summer, meaning a supporting surface, came to English via Anglo-Norman sumer, from Vulgar Latin *saumrius, from Late Latin sagmrius, a word which referred to a packsaddle, packhorse, and which was derived from Latin from sagma, packsaddle.
2. A lintel.
3. A large, heavy stone usually set on the top of a column or pilaster to support an arch or lintel.
You may, if you're a military history fan, be familiar with the idea of a sumpter mule or even a sumpter horse used as a pack animal. Sumpter, a fairly common word in Middle English, is also descended from the Old French sometier, and from Vulgar Latin *saumatrius, from Late Latin sagma, sagmat-, or packsaddle, itself from Greek, from sattein, to pack.
In other words, sommelier, summer and sumpter are all cognate, and ultimately all go back to some version of Latin sagma, and further back, to Greek sattein "to pack." Sommelier, summer and sumpter all have something to do with the business of packing or transporting, whether it's wine, or weight bearing loads, or supplies and goods.
[I wrote an earlier version of this post for another site that appears to be no longer online]