Is there a word for "an only child"?

Solution 1:

I don't think there is a single term for that, only child is the more common expression used in the English language:

• In a family with multiple offspring, first-borns may be briefly considered only children and have a similar early family environment, but the term only child is generally applied only to those individuals who never have siblings.

(Wikipedia)

Ngram shows usages of the expression an only child from the late 17th century, but it is probably older.

Solution 2:

Consider,

oneling

: (rare) a singleton; maverick; a single or only child.

From one +‎ -ling. Cognate with Dutch eenling, German Einling. Wikipedia

1823, John Cole, Herveiana: Considering how she was humoured, when a oneling, I think her behaviour is extraordinary. engYes

It's possessed by the mother, and it emphasizes the mother, that she has only one child. If you"d want to emphasize the child is oneling, your attempt would fit better WordReference - Discussion in 'עברית (Hebrew)' started by hadronic, Aug 18, 2014.

singleton

a. a child or animal that is the only one born at one birth

b. an only child in a family Random House

Solution 3:

You could try onlyborn - while it is similar to the two words you listed as not being interested in, it is a single word and is the only synonym I could find. Google returns only 1450 results using it, but Wikitionary's citation page notes multiple uses of it in books on parenting and the teenage years. It does not seem particularly archaic, given that the most recent use was in 2001.

Solution 4:

Only is occasionally used as a noun meaning "an only child":

Most adult onlys acknowledge that there are definite advantages to their only-child status. [link]

No wonder that parents say, on population surveys, that one of the most important reasons for having a second child is to prevent the first from becoming an only. [link]

She was an only child, as I was an only child, and my father was an only child, too: a family of onlys. [link]

A child who has siblings is more subject to control than is an Only. [link]

She found, for example, that onlys are more likely to have been read aloud to […] [link]

That said, this use is fairly marginal; it is basically a sort of a shorthand for only child, usually used only when the topic is already quite clear from context.

Solution 5:

What is obscure is why English doesn't have a word for an 'only child'. The language of child-bearing and childlessness is fairly rich but there is a glaring absence of language at this point.

If we take 'progeny' as a starting point, referencing the OED:

progeny
[ME. a. obs. F. progenie (13th c. in Godef.), ad. L. prōgeniē-s descent, family, offspring, f. prōgign-ĕre to beget.]
1. a. The offspring (of a father or mother, or of both); issue, children collectively; more widely, descendants. (Rarely with indef. art.)
a 1300 Cursor M. 1361 Til him and til his progeni Wit pite sal he sceu his merci. a 1325 in Horstm. Altengl. Leg. (1878) 145 Þo Eue wist sche schuld dye, Sche cleped forþ hir progenie. c 1386 Chaucer Pars. T. ⁋250 He moste nedes dye..and al his progenye in this world. 1515 Barclay Egloges (1570) C vj b/1 In it remayneth the worthy gouernour, A stocke and fountayne of noble progeny. c 1586 C'tess Pembroke Ps. cv. ii, His servantes you, O Abrahams progeny. 1604 Bk. Com. Prayer, Pr. for R. Family, All the King and Queenes Royall progenie. a 1618 Raleigh Mahomet (1637) 26 The Mores are the progeny of such Arabians as after their Conquests seated themself in that part of Affrica. 1727 De Foe Syst. Magic i. i. (1840) 13 Some think..that Noah's sons..were saved in the ark..merely for being the posterity or progeny of a righteous father. 1860 Hawthorne Marb. Faun xxvi, From this union sprang a vigorous progeny.

My Latin is a little rougher than I thought (thanks Sumelic), so my initial concept that the singular of progeny is 'progenae' is pure hokum. This takes us back to progeny or perhaps progenie in the singular sense. All this achieves, however, is to suggest a singular instance of (potentially) many progeny.

Latin again might provide a starting point with the word 'suboles' which essentially means 'shoot', 'scion' , 'sprout' or 'offspring'. From this we might come to 'unum suboles' which points directly to 'only child', but it is still two words.

The closest this approach takes us to a single word description for an only child is 'monoprogeny', or more readily, monoprog. Apart from being examples of egregious blending of Greek and Latin elements, these terms do not recommend themselves in that they have virtually no currency (the closest: http://www.waywordradio.org/monoprogeny_1/), although 'prog' is a slang expression for child, in Australia at least. EL&U member Sumelic's thoughts on this matter point me in the direction of 'uniprog' which is at least consistently Latin (though not of a reputable sort), but I'm not at all sure that he/she would like to lay claim to this bastard child.