The long face of a thing
In present-day English, side is the lateral surface of an object, the left or right half of a body, a party in an argument or a match, one aspect of a problem, and, in American restaurants, a dish that arrives with the main course. The spatial sense is still the core: a thing has length, and one of the long faces is a side. Almost every later use is that geometry, stretched.
The origin of the word side is not a French or Latin loan. English inherited it.
Already English before the Conquest
Old English had a feminine noun sīde. Glossaries and medical writing use it for the flank of a living body, matching Latin latus. The same form could name the wall of a house or the hull of a ship, a direction (day on one side of the earth, night on the other), and even a line of kin: people born of a ruler’s sīde. Those uses sit in texts from before 1066. The Oxford English Dictionary places the noun in the Old English period, before 1150; other standard dictionaries date it before 900.
It is a West Germanic inheritance, not a medieval import. Sister forms run through the family: Old Saxon sīda, Old Frisian sīde, Old Dutch sīda (later Dutch zijde and the clipped zij), Old High German sīta (modern German Seite), and, in the north, Old Norse síða, which yields Icelandic síða, Swedish sida, and Danish and Norwegian side. Gothic, unusually, has left no matching noun. That silence is a genuine gap in the record; it does not make the word late everywhere else.
One Middle English spelling siðe, in a poem whose language points to west Norfolk, may show Scandinavian coloring, or it may be a scribe’s slip. Either way, English did not need to borrow the noun. It already owned it.
From long to flank
Comparativists reconstruct a Proto-Germanic noun sīdǭ (also written sīdō or sīdōn- in different handbooks). They treat it as a derivative of an adjective sīdaz meaning long. That adjective survived in Old English as sīd, used of things that were long, broad, or spacious, and in Old Norse as siðr, said of hair or clothing that hung down. The step sources propose, without claiming a single attested scene, is from length to the long surface of an object, and from there to the flank of a person or an animal.
How far back the adjective itself goes is less settled. One widely used etymological dictionary ties it to a Proto-Indo-European root se- with the senses long and late, a family that later, through Latin and French, also gave English soiree. Wiktionary prefers seh₁-, long or lasting. Other desk dictionaries sketch still different Indo-European bases, including one that links the group to a root for throwing or letting go. The Germanic bond between long and side is firm. The exact Indo-European formula is not. A tidy deeper story would be a guess.
Morphologically the English noun is plain: no prefix a modern speaker can peel off, no living suffix. The related Old English adjective in the spacious sense has died. What remains is the noun, plus a later adjective side that came to mean coming from the flank and then subordinate, as in a side road or a side trip.
How the senses fanned out
The body sense never left. It is still the ache under the ribs, and from the fourteenth century it is also the lateral half of a slaughtered animal, the side of bacon measured against the bone. Cutting meat into sides is the first verbal use, late in the fifteenth century.
By about 1200, side by side already meant two bodies or objects placed with their flanks together. Around 1400 the noun could name a district: a South Side, a stretch of countryside, the long run of land rather than a single edge. Hillside still carries that older idea of the long part of a thing. In the sixteenth and seventeenth centuries a side-coat was simply a long coat, another fossil of the same thought.
The figurative jump is earlier than many speakers expect. By the middle of the thirteenth century a side could be a stance: a group of people set against another group, the ancestor of choosing sides and of one side of a story. Late in the fourteenth century it is a party to a deal. By the middle of the fifteenth it is an aspect of something that has no body at all, the bright side of a situation. Sporting teams as sides wait until the 1690s. British English still uses the word for a football or rugby club in a way American English usually does not.
To side with someone, to take a position in a dispute, is recorded around 1600, after an earlier idiom hold sides. Take sides follows about a century later. The restaurant sense of a dish on the side is American English of the 1880s; the slang on the side for something extra and often unacknowledged is a little later, from 1893. A phonograph side, one face of a record, appears in 1936.
None of these later meanings required a new etymon. They are the old flank, reused.
What the sisters still show
German Seite and Dutch zijde keep a similar range: a page, a party, a flank. Old Norse síða could mean a coast as well as a side of meat, another long edge of land. An English speaker who meets Seite in a German classroom is looking at a sister word, not a loan.
The useful dating is small and concrete. The political and moral side is medieval, not modern. Long before team lists and vinyl, English already used the word for a posture in a quarrel. The body came first. The faction followed.