A piece of almost everything
In modern English, part names a portion of a whole, a component inside a machine, a share of work or profit, a role in a play, a line of music, or, in American usage, the line that splits a head of hair. The noun is short and ordinary; the history is not. English did not inherit this word from its Germanic stock. It took it from Latin, and it took it more than once.
The origin of the word part is a borrowing with two English landings rather than a single clean import.
Two roads into English
Dictionaries do not quite agree on the first date because they are not always describing the same arrival. One standard account places the noun in English before the twelfth century. Another treats the familiar modern noun as a mid-thirteenth-century word. Those dates sit together once the path is split.
Late Old English already had part, borrowed from Latin partem, the accusative of pars. One specialized use, the grammatical label for a part of speech, did not last. The form that filled Middle English and then modern English came in again through Old French part, itself from that same Latin accusative. A historical dictionary of English therefore lists mixed origins: partly Latin, partly French.
French part already covered a share, a portion, a character, a side, even a path. Middle English kept the shape and widened it. Native English already had words for a slice of a whole, notably deal from Old English dæl, and also dole from dal. Part crowded most of those senses aside. Deal survived with a narrower life: a quantity, a transaction, an arrangement. Dole kept the sense of a portion handed out.
The verb arrived on a related track. Around 1200, Middle English parten meant to leave. That came from Old French partir, to divide or separate, from Latin partire or partiri, to share, distribute, or divide, built on the same noun pars. Only later did English use the verb for causing people or things to split, for sharing, for combing hair into two directions, and for giving something up. The phrase part with first meant to share with someone and later to surrender it.
Latin pars and an uncertain root
Latin pars, genitive partis, was a third-declension feminine. It already meant more than a fragment. A speaker could use it for a piece of a body, a fraction, a region, a duty or office, a theatrical role, a political faction, even a lot or fate. English later picked up several of those Latin senses one by one rather than inventing them from scratch.
The immediate ancestor is reconstructed as Proto-Italic *partis. After that, the trail forks. Some handbooks attach Latin pars to a Proto-Indo-European root *pere- or *perə- with the sense to grant or allot. On that account, a part is what has been assigned. The same root is used to explain Latin portio, a share, and a family of English words that grew from pars: particle, partial, partition, participate, party, parcel. Distant comparisons sometimes cited on this line include Hittite parshiya- for a fraction or part, Sanskrit purtam for a reward, and a Greek form meaning that something has been granted.
Other etymologists are less sure. One proposal links the Latin noun to a root meaning sell or exchange. Another points to a root meaning pass through. A specialist Latin etymological dictionary prefers a connection with Latin pario, to bring forth. These are competing reconstructions, not settled facts. The Latin and Romance history is solid; the Indo-European step is not.
What is not in dispute is the Romance spread. Italian, Spanish, Portuguese, and Romanian have parte; Catalan and French keep part; Irish took páirt as a later loan. English sits in that company as a borrower, not as a Germanic cognate of those forms.
Senses that piled up
Once the noun was established in Middle English, meaning grew by occupation rather than by sudden invention.
By about 1300 it could mean an allotted share. By the late fourteenth century it named a share of action or influence: a role, a duty. That is the period of take part and for the most part. Late in the fifteenth century, an actor’s share in a performance became a part in the theatrical sense; Latin already used a plural of pars that way. Music followed in the 1520s: one voice or instrument inside a concerted piece. A separate piece of a machine is later still, from the early nineteenth century.
The hair sense is American English of about 1890. British English more often says parting. English had older native words for that line, including Old English scead, which gave Middle English shede. The verb to part the hair is older than the American noun, attested from the 1610s.
The adjective is later than the noun, from the 1590s. None of these later uses needed a new borrowing; they are English stretching a Latin share-word across new domains.
What the replacement still shows
The most concrete leftover of this history is not a fossil spelling but a rivalry that English almost forgot. When speakers needed a word for a piece of a whole, they once had deal. Latin pars, arriving through French and an earlier Old English loan, took that job. Deal did not vanish; it retreated into quantity and bargain. Part kept the older Latin range: a fragment, a role, a side, a duty. That is why an actor still plays a part, a citizen still takes part, and a machine still needs a part: three English uses that were already available, in outline, in pars.