What the word does now
In present-day English, inside is the ordinary name for an interior: the space within a building, the inner face of a wall, the hollow of a vessel. The same form works as an adverb and a preposition (go inside, inside the envelope) and as an adjective for whatever lies toward the inner surface (an inside wall, an inside lane). Informal uses pile on without changing the core idea. Plural insides still means viscera. To be inside can mean to be in prison. Inside information is knowledge held by people who already belong to a group. None of this required a foreign loan. The word is a native compound, and its earliest job was more anatomical than architectural.
Built in English
The origin of the word inside is a Middle English compound, not a borrowing from French or Latin. Independent references agree on the pieces: in plus side. The older spelling ynneside, also written with inne-, is the form that medieval medical English actually used. The Middle English Dictionary cites a late-fourteenth-century surgical compilation, surviving in a later fifteenth-century copy, in which ynneside means the interior of the body. Medicine that never reaches that far is of little use. That bodily reading is the first sense most etymological surveys still give for the noun.
The first-date story is not quite settled. Some dictionaries place the compound in the late 1300s, in a window of about 1350–1400, which fits that medical evidence. The Oxford English Dictionary, working from the citations it accepts, gives its earliest evidence as 1504 and treats the first securely dated use as early sixteenth century. The disagreement is about which texts count as first evidence, not about two rival etymologies. The OED labels the first element an adjective in; other standard accounts treat it as the preposition. In practice those labels describe the same short native word.
Around 1500 the noun widened. It no longer had to mean flesh and organs. Any inner surface or inner part of a thing could be an inside: the inner face of a wall, the hollow of a cup, the space within a house. From that general interior the later parts of speech grow.
The two halves
Both elements are inherited Germanic vocabulary.
Old English already had a heavy-duty in. The preposition covered in, into, upon, on, at, among, and stretches of time. A related adverb inne meant within. In Middle English the shorter form took over much of that adverbial work as well. Behind both sits Proto-Germanic in, the source also of Old Frisian, Dutch, German, and Gothic in and of Old Norse í. That Germanic form is reconstructed from a Proto-Indo-European root en with the same spatial sense. English did not need Latin interior in order to talk about being in something.
Side is Old English sīde, first the flank of a person or animal, then the long aspect of an object, a coast, or a direction. It comes from Proto-Germanic *sīdō (also reconstructed *sīdǭ), a noun built on an adjective meaning long, broad, or hanging down. Cognates include German Seite, Dutch zijde, Swedish sida, Danish side, and Old Norse síða. A plausible path from long to side is the long surface of a trunk or of a thing.
Put together, the compound is transparent. The in-side is the flank or surface that lies within. Other Germanic languages built the same idea independently: German Innenseite, Danish inderside, Swedish insida, Dutch binnenzijde, and similar Low German and Frisian forms. Those are sibling compounds, not one ancestral Proto-Germanic word that English merely continued. Each language glued its own word for in to its own word for side.
From anatomy to advantage
Once the noun existed, grammar followed. An adjective meaning situated on the inner side is recorded from the 1610s. One widely used American dictionary places the adverb in 1803 and the preposition in 1791, even though everyday spatial talk with the word was already old. American English also developed inside of for a span of time, attested from 1839.
The first, bodily sense did not disappear. Plural insides remains a colloquial word for guts. A dated British use treated an inside as a passenger riding within a coach, as opposed to someone perched outside on the box. The contrast is physical and social at once: shelter versus exposure.
In the nineteenth and twentieth centuries the inner position became a metaphor for a shorter path and for restricted knowledge. Horse racing supplied the image. On a curved track the inner lane is shorter than the outer ones; inside track is recorded as a racing term around 1830 and as a general name for a competitive edge by 1854. American English has inside job by 1887 for a theft or leak done with help from someone who already belongs to the place; in the same period the phrase could also mean indoor work. Inside man appears in 1911, first for a worker used by management to watch union activity. Around 1900 slang used inside of facts that only an insider would know. Baseball kept a more literal geometry: an inside pitch crosses the plate toward the batter.
Prison slang compressed location into a single adverb. To go inside is to be locked up. That use sits beside the older, blander adverb of staying indoors during rain. The word never needed a new root for any of these jobs. It kept pointing at the inner surface and let context decide whether that surface was a wall, a gut, a racetrack, a workplace, or a cell.
A check from the neighbors
German Innenseite is a useful control on the English story. English did not import that German noun, and German did not import the English one. Both languages, and several of their neighbors, independently joined a word for in to a word for side. The dating quarrel between a fourteenth-century medical ynneside and a 1504 citation is real, and it is worth stating as a quarrel rather than smoothing it away. It does not hide the formation. Inside is a home-grown compound whose first recorded work was anatomical, and whose later work—time limits, secrecy, the short lane on a curve—still rests on that same inner surface.