This has the melancholy distinction of being one of the very few words in English that derive from the Cornish language (which died out in the late eighteenth century, though enthusiasts have revived it). It is mainly a term in archaeology for a kind of man-made stone-lined underground passage found in Cornwall. Some are rather large, opening out from a narrow entrance to a long chamber that can sometimes hold a substantial group of people. They are of Iron Age date and seem always to have been built on the edge of settlements.
Nobody knows what they were for. They can hardly have functioned as defensive structures, because they would have been a death trap for anybody sheltering inside (unlike underground structures elsewhere, which do seem to have been designed as hiding places); they are too big and too damp to be used for storage; they’re the wrong shape and their entrances are often too narrow for housing livestock. Some people believe that they are ritual structures, perhaps associated with an earth-mother religion or sun worship (some seem to be aligned with midsummer sunrise or sunset).
The Cornish word from which their name comes originally meant a cave. They have also been known as vugs or vows, from the related Cornish vooga for a cavern. Dictionaries usually say that fogou is pronounced /ˈfəʊɡuː/ , but archaeologists seem to prefer /ˈfuːɡuː/.