‘SOMEONE’S LEFT THEIR BOOK ON THE TABLE’
‘EINHVER HEFUR GLEYMT BÓKINNI SINNI Á BORÐINU’
The ‘epicene’ (= denoting both masculine and feminine) use of they, them, their, themselves, themself, theirselves, theirself) in the singular is recorded as early as the 15th century in the Oxford English Dictionary:
2. Often used in reference to a singular noun made universal by every, any, no, etc., or applicable to one of either sex (= he or she).
1526Pilgr. Perf. (W. de W. 1531) 163 b, Yf... a psalme scape ony persone, or a lesson, or else yt they omyt one verse or twayne.
1535FisherWays perf. Relig. ix. Wks. (1876) 383 He neuer forsaketh any creature vnlesse they before haue forsaken them selues.
1749FieldingTom Jones viii. xi, Every Body fell a laughing, as how could they help it.
1759ChesterfieldLett. IV. ccclv. 170 If a person is born of a..gloomy temper..they cannot help it.
1835Whewell in Life (1881) 173 Nobody can deprive us of the Church, if they would.
1858BagehotLit. Stud. (1879) II. 206 Nobody fancies for a moment that they are reading about anything beyond the pale of ordinary propriety.
1866RuskinCrown Wild Olives ß38 (1873) 44 Now, nobody does anything well that they cannot help doing.
1874 [see themselves 5].
Here is the OED on themselves. Interestingly, I had to turn offthe AutoCorrect function in Office 2000 in order to be allowed to write the word ‘themself’, which the OED records from the 15th cent .
5. In concord with a singular pronoun or n. denoting a person, in cases where the meaning implies more than one, as when the n. is qualified by a distributive, or refers to either sex: = himself or herself. Cf. they 2, them 2.
a.1464Rolls of Parlt. V. 513/2 Inheritements, of which any of the seid persones..was seised by theym self, or joyntly with other.
—1489CaxtonSonnes of Aymon i. 39 Eche of theym sholde..make theymselfe redy.
1533MoreApol. 55 b, Neyther Tyndale there nor thys precher..hath by theyr maner of expounynge..wonne them self mych wurshyp.
c.1600Shakes.Lucr. 125 Euery one to rest themselues [ed. 1594 himselfe] betake.
1654-66Earl of OrreryParthen. (1676) 147 All that happened, which every one assured themselves, would render him a large sharer in the general joy.
1874G. W. DasentHalf a Life 3 Every one likes to keep it to themselves as long as they can
Since gender-awareness this usage has been foregrounded. The New Oxford Dictionary of English (1998) says:
The word they (with its counterparts them, their and themselves as a singular pronoun to refer to a person of unspecified sex has been in use at least since the 16th century. In the late 20th century, as the traditional use of he to refer to a person of either sex came under scrutiny on the grounds of sexism, this use of they has become more common. ... Sentences such as ask a friend if they could help are still criticized for being ungrammatical. Nevertheless, in view of the growing acceptance of they and its obvious practical advantages, they is used in many places in this dictionary where he would have been used formerly.
We have no evidence of use of epicene pronouns in OE but I'd like to point out the a partially parallel development, the Icelandic use of the neuter plural þau for mixed-gender groups, is normal in Old Norse as in Modern Icelandic. ON and Modern Icelandic also use the same technique for mixed-gender agreement problems. Compare for instance French
Le garçon est paresseux[MASC.SG] ‘the boy is lazy’
La fille est paresseuse[FEM.SG] ‘the girl is lazy’
Les garçons sont paresseux[MASC.PL.] ‘the boys are lazy’
Les filles sont paresseuses[FEM: PL.] ‘the girls are lazy’
Le garçon et la fille sont tous les deux paresseaux[MASC.PL] ‘the boy are the girl are both lazy’
Icelandic:
Strákurinn er latur[MASC.SG] ‘the boy is lazy’
Stúlkan er löt [FEM.SG.] ‘the girl is lazy+
Strákurinn og stúlkan eru bæði[NEUT.PL] löt[NEUT.PL] ‘the boy and the girl are both lazy.