(adj.) set apart, detached; alone
For such a familiar word, it might be surprising to find that English speakers have only be talking about isolation and being isolated since the mid 1700s. (The verb isolate, moreover, is an even more recent invention, formed in reverse by shortening the adjective isolated in the early 1800s.)
We borrowed isolated from French, and for a long time it was treated as a foreign word in English; it was not fully naturalized until long into the nineteenth century:
The events we are witnesses of in the course of the longest life appear to us very often original, unprepared, single, unrelative, if I may use such a word for want of a better. In French, I would say, isolés.
Lord Bolingbroke, c. 1751
French took its word isolé from the Italian isolato, and it in turn comes from the Latin adjective insulatus. At its root is the Latin word for an island, insula, to which has been added a suffix, –atus, used to form adjectives from nouns. Insulatus—and, at some length, our word isolated—ultimately means ‘made into an island’.
That root word insula is also the origin of several other familiar English words, like insulated (which also means ‘made into an island’), peninsula (which means ‘almost an island’), and insulin (so-called because it’s produced in cells called ‘islets of Langerhans’). It’s also the origin of our word isle, which came to us via the French île. But it is not, weirdly, the origin of the word island itself.
While isle has classical French and Latin roots, island is decidedly Germanic. At its core is an Old English word for an island, eg or ieg (roughly pronounced like the vowel sounds in play and plié respectively), the roots of which can probably be traced back to an even more ancient word for water or a river.
In Old English eg was often used not just of islands entirely surrounded by water, but of places almost cut off or detached from a mainland or from their surroundings—like a loop of land in the course of a river, an area of high land in a flat landscape, or a patch of dry, habitable land in the middle of bog or marshland.
We can see this use of eg play out in English place names. The village of Athelney in Somerset, for instance, stands on a dry patch of land in the middle of the Somerset Levels, the majority of which lies below sea level; its name means ‘island of the prince’. The city of Ely, in Cambridgeshire, is so called because it stands on an expanse of solid clay in the middle of the otherwise waterlogged Fenlands; its name is popularly said to mean ‘eel-island’. And the town of Romsey in Hampshire was once a metaphorical ‘island’ because it was once the site of a closed monastic retreat; its name is said to derive from the island of some local figure known as Rum, or Rumwald.
Eg plus land in Old English, meanwhile, gave us the word island. But with English also picking up isle from French, confusion between the two became inevitable. People eventually forgot the original Old English morphology and meaning of eg, wrongly assumed that it must share the same origin as isle, and began adding an S to it to make the two words appear as cousins—despite the fact that they developed in isolation.