Older, higher, last in line
In current English, senior marks someone older than another person, higher in rank, or further along in a course of study. A father may be listed as John Smith Senior when his son has the same given name. An officer may be senior to the rest of a staff. A student in the final year of high school or college is a senior. Used as a noun, the word can also mean an older adult. Those jobs look scattered until they are read as later uses of one Latin comparative that English borrowed ready-made.
The origin of the word senior is a direct loan from Latin senior, older or elder. That form is the comparative of senex, an adjective and noun for an old person, with genitive senis. English did not grow the word from native Germanic stock. Old English already had elder and eldest for comparing age, and those forms still survive in limited use. Senior arrived as a finished Latin comparative and kept that shape.
A name-tag before a rank
The earliest English evidence is from the late thirteenth century. The word then appears after personal names to mark the father when two men share a name. A frequently cited civic record from the Leet rolls of Norwich, in the late 1200s, attaches the Latin comparative to a man named Walterus Baddyng. General dictionaries do not all agree on the first English date. Some place first use in the fourteenth century, and at least one major American dictionary waits until the fifteenth. Treatments that cite the Norwich rolls put the adjective in the late thirteenth century. The function itself is not in dispute: the word began as a distinguisher of age within a family, not as a job title.
The noun followed the adjective. By the middle of the fourteenth century it could mean a person of authority. By the late fourteenth it could mean simply someone older than someone else. In biblical English from the fourteenth to the sixteenth centuries it also served for an elder or presbyter, a sense that later dropped out of ordinary use. Rank and length of service come later still. The meaning higher in office or longer in a post is recorded from the early sixteenth century. That is the path from father-versus-son to senior partner and senior officer.
The comparative English kept whole
Latin formed senior with the regular comparative ending on senex. English speakers took the whole form rather than adding a native comparative to a borrowed base. The deeper source is the Proto-Indo-European root reconstructed as sen- or senos, meaning old. That root is unusually stable across the family. Sanskrit has sana- for old. Lithuanian has senas. Old Irish sen and Old Welsh hen keep the same sense. Armenian hin does too. Gothic sineigs was used of old people, not of old objects. Greek henos could mean of last year.
Latin itself split the idea of age. Senex applied mainly to living beings, especially people. Vetus, literally having many years, was the usual word for old things. That person-centered use of senex is why so many Latin derivatives of the same stem concern human age and human councils. Senate comes from senatus, a council of elders. Senile comes from senilis. Seniority is an abstract formed later, from medieval Latin. Those are relatives of senior, not sources of it.
The morphology is transparent in Latin and only half-transparent in English. The stem sen- and the comparative suffix -ior are real pieces of the Latin word. English does not treat senior as a stem plus a living suffix; there is no native series sen, sener, senest. The antonym junior is a matching Latin comparative, from iuvenis, young. English borrowed both comparatives as closed pairs rather than building them at home.
Doublets on the Romance road
The same Latin comparative did not travel into English only once. In spoken Latin and then in the Romance languages it became a title of respect: a lord, later a polite form of address. French seigneur and sire, Spanish señor, Portuguese senhor, and Italian signore all go back to Latin senior, often through medieval senses of feudal lord, husband, or abbot. English later borrowed several of those as well. Senior, sir, sire, seigneur, and señor are therefore doublets: different later shapes of one Latin word. Sir is the everyday English outcome of the Old French title path. Senior is the learned Latin shape that never became a form of address in English.
Roman usage already had a civic sense that English did not need to invent. In the census and the army, the seniores were the older men, above ordinary military age, often given as over forty-five. Medieval Latin then stretched the noun toward rank. English took the comparative as a relative marker of age and only later reattached rank.
School years and a polite label for age
Once the word meant more advanced, schools could use it. From the early seventeenth century a senior could be an advanced student. In American usage the noun and adjective settled on the last year of high school or college. Sources differ on when the fourth-year sense became fixed. Some put a general college sense in the seventeenth century and a clearly fourth-year reading much later, even in the 1880s. Others date American fourth-year use earlier, in the eighteenth century. What is stable is the pairing with junior, which English also took from Latin.
The noun for an elderly inhabitant is recorded from the late nineteenth century. Senior citizen, typically a person past working age, is American English from 1938. That phrase keeps the Latin comparative while softening the bluntness of old. It is also a reminder that the English word never lost its first job: comparing one person with another.
The native pair elder and eldest still does some of the same work, especially in families and churches. Senior remains the Latin loan that English kept for names, offices, classrooms, and, later, a courteous way to talk about age. Sanskrit sana- and Irish sen show that the comparison is much older than any of those English uses. The root was already the ordinary word for old long before Latin made a comparative and English borrowed it whole.