A person, not a practice
A monogamist is someone who keeps, or argues for, a single partner at a time: in a marriage, in a long relationship, or, in looser modern talk, for the length of one exclusive stretch after another. The noun names the person. The related abstract, monogamy, names the arrangement. That split is ordinary English word-building, but the Greek compounds behind both words were about something narrower than today’s dating vocabulary.
The origin of the word monogamist is English on the surface. Speakers took monogamy and added the agent suffix -ist, the same ending that turns a practice or doctrine into a person who holds it. Historical dictionaries do not lock down a single first date. Several place the noun in the mid-seventeenth century, in a window around 1645 to 1651. Merriam-Webster is more cautious and dates the familiar sense to about 1721. Either way, the person-noun appears a generation or two after the abstract noun it was built from.
How the noun reached English
Monogamy itself is a learned borrowing of the early 1600s. English took it from French monogamie. That French form restates Late Latin monogamia, which restates Ancient Greek monogamia, “single marriage,” built on the adjective monogamos, “marrying only once.” Merriam-Webster dates English monogamy to 1612; other etymological surveys put the same word in the 1610s. The path is borrowing, not native inheritance from Old English. Nothing in the everyday Germanic stock of marry, wed, or wife supplied this particular compound.
The same scholarly route produced a small family of English relatives. Polygamy and bigamy travel through Latin and French on the same Greek model, with poly- “many” and bi- “two” in place of mono-. A monogamist, a polygamist, and a bigamist are agent nouns of the same type: a person defined by how many marriages, or how many living spouses, the speaker has in view. French still has monogamie and the adjective monogame; German has Monogamie and Monogamist. Modern Greek keeps μονογαμία. Those are parallel learned forms, not folk descendants of a village word.
The two Greek halves, and the suffix
The Greek compound is transparent once it is taken apart. The first half is monos, “single, alone.” That adjective is the source of the combining form English writes as mono- in a long list of technical words. Historical linguists connect Greek monos with a Proto-Indo-European root reconstructed as *men- in the sense “small, isolated.” Outside Greek, Armenian manr, “thin, slender, small,” is often cited in the same family. Through other Greek descendants of the same adjective, English also has monk and monastery: a person set apart, and the place for that setting-apart. The “one” in monogamist is that monos, not the native English numeral.
The second half is gamos, “marriage, wedding, the married state.” It belongs with the verb gamein, “to marry, to take a wife,” and with older Greek gametes and gamete in the senses “husband” and “wife.” The deeper reconstruction is a Proto-Indo-European root *gem- or *ǵem-, “to marry.” Relatives outside Greek are mostly kinship terms, not full marriage nouns: Sanskrit forms in the set around brother, sister, and daughter-in-law; Avestan zama-tar, “son-in-law”; Latin gener, also “son-in-law.” English did not inherit those as everyday kin words. It met gamos through the scholarly combining forms -gamy and -gamous.
The suffix on the English person-noun is a separate layer. English -ist comes through Middle English and Old French -iste from Latin -ista, itself from Greek -istes, an agent ending. So the full word is a Greek marriage compound, filtered through Late Latin and French as an abstract, then given a Greco-Latin person-suffix that English had already made native. Surface analysis as monogamy plus -ist is correct for the last step. It is not the whole history.
Marrying once, then marrying one
The Greek and early modern sense is easy to miss. Monogamos did not first mean faithful while married. It meant marrying only once in a lifetime: one wedding, and no second after a spouse died. That is the sense English first borrowed for monogamy in the 1610s. The contrast was not only with polygamy, several spouses at once, but with remarriage after widowhood. American Heritage still lists that lifetime-once reading among the definitions of monogamy, though it is no longer the default.
By 1708 the abstract noun is recorded for the later, more familiar meaning: being married to only one person at a time. The person-noun followed that shift. A monogamist could be someone who practiced or upheld that one-at-a-time rule, not necessarily someone who refused a second marriage after a death.
An eighteenth-century literary use still holds the older theological sense. In Oliver Goldsmith’s 1766 novel The Vicar of Wakefield, the narrator sides with William Whiston’s view that an Anglican priest should not take a second wife after the death of the first, and he calls himself a strict monogamist on that ground. The scene is useful because it shows the word still doing clerical argument, not lifestyle branding. Etymonline notes that monogamous, recorded later in the eighteenth century, could likewise mean “not remarrying after the death of a spouse,” alongside the one-living-spouse sense.
Later English stretched the family again. Zoology adopted monogamous for animals with a single mate in a season or for life. Everyday speech added serial monogamist for someone who moves from one exclusive partnership to the next without overlapping them. That last use keeps the “one at a time” reading and drops both the lifetime ban on remarriage and, often, the requirement of legal marriage. The agent noun did not need a new etymology for those senses; the social meaning of “one” changed around it.
One dating quarrel worth keeping
The pieces of the word never hid. Mono- still means one; -gam- still means marriage; -ist still means a person. What changed is which “one” speakers meant: one wedding in a life, one living spouse at a time, or one partner for now. The disagreement over first English dates is a reminder that first-attestation notes are estimates, not a single archive stamp. Some records cluster around 1650; at least one major dictionary waits until about 1721. What is firm is the path: Greek monogamos, Late Latin and French monogamia and monogamie, English monogamy in the 1610s, then the person who lives by that “one,” or argues for it, as a monogamist.