The word is from Anglo-Normanbacheler (later suffixal change to bachelier; cf. escolier "student", from earlier escoler), a young squire in training. The ultimate source of the word is uncertain, it may be from Medieval Latinbaccalari(u)s "vassal farmer" or "farm hand" (cf. Provençalbacalar, Tuscan bacalaro "squire"), i.e. one who tends a baccalaria, a term for a grazing farm (from bacca "cow"), or it may be from Latin baculum "a stick" (as the knight-in-training would practice with a wooden club before receiving his sword).
The Old French term crossed into English around 1300, referring to one belonging to the lowest stage of knighthood. Knights bachelor were either poor vassals who could not afford to take the field under their own banner, or knights too young to support the responsibility and dignity of knights banneret.
From the 14th century, the term was also used for a junior member of a guild (otherwise known as "yeomen") or university; hence, an ecclesiastic of an inferior grade, for example, a young monk or even recently appointed canon; and also an inferior grade in scholarship, i.e. one holding a "bachelor's degree" In this sense the word baccalarius or baccalaureus first appears at the University of Paris in the 13th century, in the system of degrees established under the auspices of Pope Gregory IX, as applied to scholars still in statu pupillari. Thus there were two classes of baccalarii: the baccalarii cursores, theological candidates passed for admission to the divinity course; and the baccalarii dispositi, who, having completed this course, were entitled to proceed to the higher degrees.
Use for "unmarried man" in the 19th and 20th centuries
In the Victorian era, the term eligible bachelor was used in the context of upper classmatchmaking, denoting a young man who was not only unmarried and eligible for marriage, but also considered "eligible" in financial and societal terms for the prospective bride under discussion. Also in the Victorian era, the term "confirmed bachelor" denoted a man who was resolute to remain unmarried.
By the later 19th century, the term "bachelor" had acquired the general sense of "unmarried man". The expression bachelor party is recorded 1882. In 1895, a feminine equivalent "bachelor-girl" was coined, replaced in US English by the somewhat humorous "bachelorette" by the mid-1930s. After World War II, this terminology came to be seen as antiquated, and has been mostly replaced by the gender-neutral term "single" (first recorded 1964). In England and Wales, the term "bachelor" remained the official term used for the purpose of marriage registration until 2005, when it was abolished in favour of "single."
In certain Gulf Arab countries, "bachelor" can refer to men who are single as well as immigrant men married to a spouse residing in their country of origin (due to the high added cost of sponsoring a spouse onsite), and a colloquial term "executive bachelor" is also used in rental and sharing accommodation advertisements to indicate availability to white-collar bachelors in particular.
Historical examples of bachelors (men who never married)
(chronologically, by date of birth)
Medieval Period, Renaissance, and Early Enlightenment
^etymological discussion, with references: Uwe Friedrich Schmidt, Praeromanica Der Italoromania Auf Der Grundlage Des LEI (A und B), Europäische Hochschulschriften 49, 9: Italienische Sprache und Literatur, Peter Lang, 2009, 117-120.
^Severtius, de episcopis Lugdunen-sibus, p. 377, in du Cange.