An unnamed place
In current English, somewhere marks a location that is treated as real but left unspecified. Keys can be somewhere in the house. A traveler can head somewhere warmer. The same adverb also drifts off the map of space: a figure can sit somewhere between two numbers, and a date can fall somewhere about a given year. Less often, the word is used as a noun for that unspecified place itself, a somewhere rather than a named town.
The origin of the word somewhere is not a borrowing. It is an English compound, assembled from two native pieces that were already old when scribes first wrote them as a pair.
An English compound
The Oxford English Dictionary treats the form as made inside English by joining some and where. Its earliest cited evidence is from around 1175, in the Ormulum, a long Middle English verse text from the East Midlands. There the spelling is summhwær. Other historical dictionaries agree that the word belongs to late twelfth-century English, though some round the first known use to the thirteenth century. The disagreement is about how tightly to date a Middle English manuscript, not about the word’s makeup.
English did not import a ready-made indefinite locative from French or Latin and then respell it. It glued together two Germanic words it already had. Through the rest of the Middle Ages the glue was still optional on the page. Down to the end of the sixteenth century the two halves were often written as separate words. Sister compounds such as somewhat and sometime show the same delay: they are Middle English in speech, but handwriting and print keep them split well into the early modern period.
Middle English spellings wander because both halves were still in motion. Beside summhwær one finds sumwhare, sumwher, somwhere, sommewhere, and northern-looking forms with qu such as sumqwhare. The modern one-word spelling somewhere is the survivor of that crowd, not a new coinage.
What some and where already meant
Some comes from Old English sum, used for a certain person, a certain thing, or a certain indefinite quantity or number. That adjective and pronoun is inherited from Proto-Germanic sumaz, with close kin in Old Saxon, Old Frisian, and Old High German sum, Old Norse sumr, and Gothic sums. Behind those forms sits a suffixed version of the Indo-European root sem-, the same family that yields words for “one” and “together.” The vowel spelling shifted from u to o in Middle English for a scribal reason familiar from come: clusters of minims in blackletter were easier to misread, so writers preferred o.
Where is equally native. Old English had hwær and hwar, “at or in what place,” from a Proto-Germanic adverb hwar. Cognates include Old Saxon hwar, Old Norse hvar, Old Frisian hwer, Old High German hwar, modern German wo, and Gothic hvar. The wider family is the Indo-European interrogative and relative stem kwo-, the same stock that produces English who, what, and which.
So the morphology is transparent and not folk etymology. An indefinite determiner is placed in front of a locative adverb. The result means, roughly, “at an unspecified where.” English has done the same job with other bases: somehow, sometime, somewhat. The pattern is productive, but not every member of the set survived.
A family that thinned out
Somewhere kept its place. Somewhen is rare and, from the nineteenth century on, mostly heard only when speakers stack it with the commoner compounds for effect. Somewho, recorded in the late fourteenth century, did not last. Somewhy turns up now and then in the 1800s and then fades. Walter Scott used somegate in 1816 for “somewhere” or “in some way,” a northern and Scots-looking formation that never became the standard locative. American English later added someplace, written as one word by the mid-nineteenth century, as a plainer rival built on place rather than where.
English tried several indefinite some- compounds. The locative one happened to be useful enough, and transparent enough, to stay.
How the meaning moved
The first sense is still the main one: in or at a place that is left unnamed. From about 1300 the word could also mean “elsewhere,” in some other place than the one already in view. Somewhere else is on record from the late fourteenth century in some handbooks and from the mid-fifteenth in the Oxford evidence; either way, the extra word else became the usual way to mark contrast.
Motion followed location. By the early 1400s somewhere could mean “to an unspecified place,” typically with go. That directional use is ordinary now and needs no extra preposition: people go somewhere, not only sit somewhere.
A short-lived sense in the 1560s and 1570s meant “in some places; here and there.” It is rare in the record and then drops out. Another early-modern extension, from the 1630s, applies the same vagueness to books and other texts: a remark heard “somewhere” in an author, in some passage the speaker cannot pin down. The noun use, “an unspecified or indefinite place,” is cited from 1647.
Approximation is later. From the 1830s the adverb can sit beside a year or a clock time: born somewhere about a given date, waking somewhere about five. From the 1840s somewhere about is a general hedge for quantity as well as time. The modern somewhere between belongs in that same neighborhood: space grammar reused for numbers.
The best-known modern formula is newer still. In 1914, wartime datelines began to read somewhere in France (and then somewhere in Flanders, somewhere in England, and so on). Censorship and security kept exact positions off the page. The phrasing outlived the trenches as a way to name a place one will not, or cannot, specify.
The older indefinite English did not keep
German irgendwo and Dutch ergens look like natural translations, but they are not the same compound. They go back to an older Germanic construction with where plus an indefinite particle, the sort of formation seen in Old High German io (h)wergin and Middle Dutch irgen. Old English had a related word of its own, hwergin, used for “somewhere” or “anywhere,” including in the phrase elles hwergen, “somewhere else.”
That older indefinite did not become the ordinary modern word. Instead of keeping hwergin, speakers built a clearer pair from some and where, first written in the Ormulum as summhwær, and only later locked into a single spelling. English already knew how to say an unnamed place; it chose the compound that still looks like what it is.