One rather than many
In present-day English, single is the ordinary word for being one rather than several: one ticket, one occupant, one unit instead of a pair or a set. It also names a social status — not married, and in later usage not paired in a relationship — and, as a noun, a person in that state. Sports, money, and recorded music have all borrowed the same stem for a one-base hit, a one-dollar bill, and a short record or standout track. The verb, usually with out, means to pick one from a group.
None of those later trades is the starting point. The origin of the word single is a French adjective that already mixed solitude with plainness, then a Latin numeral whose usual job was to count things out one apiece.
A French loan in Middle English
English did not inherit this adjective from Old English. It arrived as a borrowing from Old French, and from the Anglo-French used in England, where the spellings sengle, sangle, saingle, and single meant alone or unaccompanied, and also simple or unadorned. Middle English kept both sengle and single. The adjective is in place by the early fourteenth century; historical lexicography finds evidence as early as 1303 in the writing of the poet and historian Robert Mannyng. That first cluster of uses already includes the marital sense: not married.
The route is borrowing, not native Germanic inheritance. That is why English single sits with simple and singular as a Latinate set, rather than with one, only, or alone, which continue older Germanic stock. The -le at the end is not a living English suffix here; it is what remained of the Latin ending after the word had passed through French.
Latin singulus and an older root for one
French continues Latin singulus. In Classical Latin the word is met chiefly in the plural, singuli, as the distributive form of the number one: one to each, one by one, a priest for each god, land measured out per person. The grammatically singular use in the sense sole or lone is uncommon in the classical language, which preferred unus, singularis, or solus for that meaning. Earlier and later Latin can show the lone sense, but the English adjective is still a mild false friend of the classical distributive.
How singulus itself was built is not settled. Most accounts attach it to the Proto-Indo-European root reconstructed as *sem-, a stem for one and for being as one, together. That same root is widely credited in Latin simplex and simplus, the source of English simple; in similis, behind similar; and, farther afield in English, in same and some. The first syllable of singulus is the same sim- / sin- element that appears in simple. Some scholars treat the Latin word as a diminutive, or as *sem- plus extra suffixes; reconstructions such as *semgolo- and *sem-go-lo- appear in the literature. Others add a distributive particle. Those inner pieces remain uncertain. What is not in serious dispute is the kinship with simple and with singular, which reached English by a parallel Latin and French route.
Through the older root rather than through the Latin word itself, related forms include Greek homos for one and the same, Sanskrit sam for together, and Old Church Slavonic samu for himself. They do not look like single on the surface, but they share the old idea of one-ness that Latin specialized into one each.
Romance descendants split the meaning in useful ways. Italian singolo and Romanian singur stay close in shape and still cover individual and, in Romanian, also alone. Spanish took another phonetic path: sendos, from the same Latin plural, still means one for each of two or more — the distributive sense that English largely left behind. A Latin diminutive *singellus gave Spanish sencillo and Portuguese singelo, simple, another reminder that this family kept splitting into simple, sole, and one-apiece.
How English piled on senses
By the late fourteenth century the English adjective also meant consisting of one unit, individual, unaccompanied, and it was often little more than an emphatic one. Undivided, of a whole not split, is recorded from the 1580s. Early modern English could call a heart single when it was sincere and unmixed, and, more rarely, call speech or matter single when it was weak or foolish. Those moral and pejorative uses have mostly died. Botany kept a narrower technical sense: a flower with one rank of petals, not a doubled ornamental bloom. The compound single-parent is much later, attested from 1966.
The noun follows the adjective. Around 1400 it could mean an unmarried person; by the mid-fifteenth century, a person alone. From the sixteenth century onward, anything that counted as one acquired specialist labels. Hunting supplied the verb in the 1570s: to cut an animal out of the herd, often with forth or out — still visible when someone is singled out. Cricket used the noun for a hit worth one run by 1851; baseball for a one-base hit by 1858, and the matching verb by 1899. American usage made a single a one-dollar bill by 1936. The record industry made it a disc with one song on each side by 1949, then a marketed standout track. The social plural singles, young unattached people as a group, and the singles bar that served them, belong to the 1960s. Singleton, an earlier modern label for an unattached person, is recorded from 1937.
The sense Latin kept elsewhere
The fact worth carrying away is the split among sister languages. Latin singuli counted one per unit. French passed English a word for alone, plain, and unmarried. English then used that word as a convenient tag for anything that comes in ones: a base, a dollar, a song. Spanish sendos, from the same Latin plural, still does the older job — one for each — which is why two guests can arrive with sendos bouquets and each still holds only one. To single someone out is the English construction that most clearly remembers that distributive past: one, taken from many.