Several jobs, one borrowed noun
In present-day English, party is unusually crowded. It can name a birthday gathering, a political organization, one side in a lawsuit, a search team, or, in older informal speech, a particular person. Those uses do not look like a single story. They are.
The origin of the word party is a French participle that meant a divided share, not a night of guests. The social sense is late. Law, faction, and the idea of a side came first.
Borrowed from French in Middle English
English took the noun in the Middle English period, around 1300. Early spellings include partie, partye, and party. The source was Anglo-Norman and Old French partie, already a noun in the twelfth century. French had turned the feminine past participle of partir, to divide or separate, into a name for a side, a portion, a share, or a division: that which has been split off.
The Oxford English Dictionary treats the English noun as a borrowing from French and lists two etymons: partie and parti. Modern French still keeps them apart. Partie is feminine and covers a part, a game, or a social occasion. Parti is masculine and names a political grouping or a chosen side. Middle English folded both into one form.
A dated landmark is about 1300, including an appearance in the Middle English legend of St. Brendan. At that stage the word often overlapped with part, a related borrowing of the same Latin stock. Party as a mere portion of a whole is now obsolete. That sense lasted into the seventeenth century and then faded. English already had part for a piece of something, so the overlapping use had little work left to do.
Latin pars, not a native suffix
Behind Old French partir stands Latin partire or partiri, to share, distribute, or divide. That verb is built on pars, with the stem part-: a part, piece, or share. The same Latin noun also gave English part, so party and part are close relatives, not accidental lookalikes.
Party is not English part plus a native suffix -y, even if that analysis looks tempting on the page. The ending is the French participle ending, fossilized as a noun. The later Italian loan partita, used in music and commerce, is a doublet from the same Latin past participle, not a child of the English word.
Etymologists often take pars further back to a Proto-Indo-European root reconstructed as a verb of granting or allotting. That deeper step is a reconstruction, not a recorded form, and dictionaries do not all present it in identical shape. What is secure is the Latin and French chain.
Romance languages keep clear descendants of the family: French partie and parti, Italian partita and partito, Spanish parte and partido. German Partei and Dutch partij are later loans of the French political noun, not independent Germanic inheritances. English never grew a native twin of this word. The older English vocabulary for a share sat closer to deal.
From a side of a dispute to a room of guests
The first English senses cluster around division. A party could be a section of a book, a fragment, a side. Almost at once, by about 1300, it named a person or group involved in a lawsuit, a contract, or an agreement: still the legal party of the first part. The same period gives people united behind a person, a policy, or a cause. That is faction before it is a modern electoral machine.
Accounts of the political sense differ in emphasis rather than in substance. One standard history already records a political grouping around 1300. Another dates the organized party that contests elections from the 1680s. The older sense is a side in a contest. The later one is a standing institution with a name, a platform, and candidates.
From the early fourteenth century the word could mean any group of people, and even a social class. By the mid-fifteenth century it could mean a particular person, a use that survives only as dated or humorous slang. Military language took it for a body of troops detached for a task. Sources disagree on the date. One places a clear military sense in the 1640s; another traces a duty-detachment meaning as early as the fourteenth century. The later date may mark a specialized use. The earlier one may simply be the general group sense applied to soldiers.
The gathering for pleasure is surprisingly late. By the early eighteenth century, around 1716, party could mean an occasion assembled for enjoyment. It grew out of older phrases for people brought together for a temporary purpose: a dinner party, a hunting party. The assigned task thinned out until the gathering itself was the point.
The verb to party, to have a good time, is twentieth-century American English, recorded in 1922. An older verb, from the 1630s, meant to take someone's side. That sense has died. The surviving verb grew from the social noun, not from the legal one.
Heraldry and property law still show the older participle. A party wall is a wall shared by two holdings: a division that belongs to both sides. The adjective there is not a decoration borrowed from birthday hats. It is the same idea of something parted.
The French split that English refused
Modern French still asks speakers to choose. A political grouping is a parti. A portion, a match, or a social occasion is a partie. English refused that split and let one noun keep recruiting jobs: litigant, faction, detachment, guest list, then the verb for staying out late.
The legal and political meanings preserve the old idea of a side. A party to a contract is one share of an agreement. A political party is a body that has taken a side. A search party is a group split off for a task. The birthday gathering looks like the odd one out only if you start from modern leisure. Historically it is the same noun after the assigned purpose dropped away, about four centuries after the word had already entered English as a name for a divided part.