(n.) a mass of water or ice suspended in the air
The word cloud derives from an Old English word, clúd, that once meant ‘hill’, or ‘mass of stone’. So a cloud was originally a rock.
Through that same root, cloud has some fairly unexpected etymological cousins in modern English, including clod (a lump of mud or earth) and clot (a congealed mass), as well as a handful of more obscure words like clout (an old word for a small piece of leather or iron, sheared from something larger), cleat (a wedge or bolt), and clew (a 1,000-year-old word for a spherical globule or a conglomeration of something, like a snowball or a ball of string).
But how does a word for a mass of rock come to be used as a word for a mass of water vapour?
It’s presumed that Old English speakers were quick to notice that thick, heavy, dark-grey rainclouds (the type anyone living in England knows an awful lot about) looked a lot like thick, heavy, dark-grey masses of stone. Consequently, Old English word clúd gained a second meaning, and by the early fourteenth century this meaning had all but replaced the older one entirely; from the Middle English period onwards, clúd (or clod, as it was spelled by then) was being used almost exclusively used to refer to clouds. It’s this meaning that has remained in use ever since.
It might seem like an odd connection, but it’s by no means alone. When the word cumulus first appeared in English in the mid-1600s, it referred to a mound or pile of something—or, in the words of the Oxford English Dictionary, to “the conical top of a heaped measure”, like a piled spoonful of flour. Etymologically, cumulus is derived from a Latin word for ‘heap’, and is a relative of words like accumulation and cumulate.
Only one question remains, then: if clúd meant ‘rock’, what was the Old English word for cloud?
The answer to that is weolcen, which is the origin of the somewhat old-fashioned English word welkin. Sadly, welkin has long since disappeared from the language almost entirely today, outside of literary contexts and a handful of local dialects, but it remained in use right up to the nineteenth century. You’ll find it in the works of Wordsworth, Longfellow, Charles Kingsley, Walter Scott and Charlotte Brontë among others, as well as in the original opening line of the Christmas favourite Hark! The Herald Angels Sing—originally a solemn and considerably un-Christmassy hymn beginning, “Hark! how all the welkin rings”.
Like clúd, however, welkin also changed its meaning over time. Although it originally meant ‘cloud’, its use broadened and grew more figurative, so that by the time Wordsworth and Brontë et al. were using it in the nineteenth century, it was taken to refer to the heavens, the firmament, the upper atmosphere, or the entirety of the sky. Likewise, to make the welkin ring, or to rend the welkin, is an old English expression describing an impossibly loud noise or cheer. Like a cloud concert. Sorry, rock concert.