What English uses it for now
In everyday English, second does more than one job. It is the ordinary ordinal after first: the next place in a line, the next year in a sequence, the runner who finishes just behind the winner. It names a rank that is not the top one, as in a second choice or a second-rate product. It is also the SI unit of time, one sixtieth of a minute, and a matching sixtieth of an angular minute in geometry. As a verb it means to support a motion in a meeting, or to stand behind someone in a contest. The uses look scattered until the older Latin sense is restored: the word is built on following.
A French loan that filled a gap
English did not inherit this form from Old English. The adjective arrived in Middle English in the late thirteenth century, with first records near 1297 and other handbooks rounding the date to about 1300. The immediate source was Old French second or secont, also found in Anglo-French as secund. Behind the French word stood Latin secundus, following or next in time or order. Some accounts allow a direct Latin influence as well, plausible in a period when clerks moved easily between French and Latin.
The borrowing belongs to the same wave of Romance vocabulary that entered English after the Norman Conquest. Native speech already had other, from Old English ōþer, which could mean second, the other of two, or another. There was also a thinner rival, twoth, formed on the numeral two the way third is formed on three. By the fifteenth century the Romance form had largely taken over the numbered slot, while other kept the contrastive work it still does today.
Scandinavian languages show what English gave up. Swedish andra and Danish anden still serve as second in many ordinary phrases, including floor numbering. German built zweiter from zwei, two. English kept two for the cardinal and imported a following-word for the ordinal.
From following to favorable
Latin secundus is not a disguised form of the numeral two. It is related to sequor, I follow, and is usually explained as an old participial or gerundival formation, reconstructed for Proto-Italic as something like sekʷondo-, that which follows. The deeper root is Proto-Indo-European sekʷ-, to follow. That is the same family that yields English sequel, sequence, consecutive, pursue, and sue.
Because something that follows can also go along with you, Latin developed a second cluster of meanings. A secundus wind or current was a following one, hence favorable. From there the adjective could mean helpful, prosperous, or subordinate. Romance descendants keep the ordinal and, in varying degrees, an according-to sense: French second, Italian secondo, Spanish and Portuguese segundo, Catalan segon, Romanian secund. French also has deuxième, built on deux, so the donor language still has a native-style rival beside the Latin one.
The morphology is transparent once the Latin verb is in view. There is no prefix meaning two. The -undus element marks a form that follows; the rest is the follow-root. Folk stories that tie the English word directly to the digit two miss that Latin already had alter and secundus as overlapping ways to say the next.
How the senses branched
Once the adjective was in place, a noun for the one next after the first appeared in the early fourteenth century. Rank senses followed: next in quality, next in authority, inferior to the leading item. Late Middle English already used it for another, the sense still audible in a second chance. Later English added more specialized ranks: second mate, second gear, and second fiddle from the supporting part in an ensemble.
The time-unit noun is the same adjective put to a technical use, not a separate root. Medieval Latin called the first sexagesimal cut of an hour the first small part, which became the minute. The next cut, one sixtieth of that minute, was the second small part, shortened to secunda. Old French passed seconde into English in the late fourteenth century, first in geometry and astronomy for a sixtieth of a minute of arc. Sources do not fully agree on when the clock sense became common: some surveys treat angular and temporal uses as arriving together in the late 1300s, while others date the everyday time unit later, even into the late sixteenth century. What is not in dispute is the naming logic. The unit is second because it is the second division, not because it is a tiny two.
The verb is later still. In the 1580s English took it from French seconder, itself from Latin secundare, to assist or make favorable. A second in a duel or a boxing match is the person who follows and supports the principal. Parliamentary procedure soon used the same verb: to second a motion is to give it the following voice it needs before debate can go on. British English later extended the idea to temporary reassignment, as when an officer is seconded to another post.
What the native words left behind
The replacement of other as an ordinal is a clean case study in why a loanword sticks. Other was already a high-frequency contrast word, a pronoun, and an adjective. Loading it with number two in a series made sentences ambiguous, especially once Middle English was absorbing French terms for ranks and law. After had done some of the same ordinal work in Old English, but it was drifting toward its modern role as a preposition, adverb, and conjunction. Second arrived with a single, sharp job.
That history still shows in odd corners. English can say every other year for a second-year rhythm without using second at all. It can also say second nature for a habit that follows so closely it feels original. Those phrases are easier to remember if secundus is heard as coming after rather than as a disguised two.
A useful test of the etymology is the Latin adjective’s extra sense of a following wind. English kept the ordinal, the time unit, and the supporting verb, and it largely dropped the meteorological metaphor. Italian secondo can still mean according to, and Spanish según preserves a related path from the same Latin word. The English noun for a sixtieth of a minute is not a random scientific coinage. It is the feminine of secundus, reused by medieval astronomers for the slice that follows the minute, then borrowed again through French. The clock hand that ticks the unit is, in the oldest available sense of the word, the one that comes next.