Companionway is one of those words I’ve seen from time to time and never bothered to look up; the general sense ‘something you walk along on a ship’ sufficed for my purposes. But in reading Jane Stevenson’s The Winter Queen (I’m on a 17th-century kick these days) I hit the line “she pointed him speechlessly towards the stairs, steep as a ship’s companionway” and realized I had a completely misleading, if vague, image of a companionway, so I looked it up. Turns out it’s (in Merriam-Webster’s words) ‘a ship’s stairway from one deck to another’; M-W says it’s from companion ‘a hood covering at the top of a companionway’ and derives that “by folk etymology from Dutch kampanje poop deck.” You mean it has nothing to do with the usual word companion? thought I—but it turns out it’s not that simple. Here’s the OED:
cf. Du. kompanje, now usually kampanje, ‘quarterdeck’ (i.e. above the cabin in the old ships of the line), … corresp. to OF. compagne ‘chambre du majordome d’une galère’ (Littré), It. compagna, more fully chambre de la compagne, camera della compagna, … from It. and med.L. compagna, … ‘vivres, provisions de bouche’ (Jal).
The (camera della) Compagna was thus originally the pantry or store-room of provisions in the mediæval galley, found already in 14th c. Pantero-Pantera, Armata Navale (Rome 1613) iv. 45, describes it as ‘la camera della Campagna, che serve come una dispensa, nella quale sta il vino, il companatico, cioè carne salata, il formaggio, l’oglio, l’aceto, i salumi, e l’altre robbe simili’ (Jal). The name has passed in Du. and Eng. to other structures erected on the deck. In Eng. corrupted by sailors into conformity with COMPANION1 (to which it is indeed related in origin).
So a Vulgar Latin word meaning ‘what one eats with bread’ (cum pane) becomes a Romance word for ‘provisions’ and thus (via a phrase ‘room for provisions, ship’s storeroom’) to a particular cabin and then the deck associated with it, but its Dutch form kompanje sounded enough like the word for ‘someone who shares your bread with you’ that English sailors pulled it back into that form. Lovely! (But why does M-W ignore this backstory and leave the word’s history at the Dutch phase?)