they

Etymology

From Middle English þei, borrowed in the 1200s from Old Norse þeir, plural of the demonstrative sá which acted as a plural pronoun. Displaced native Middle English he from Old English hīe — which vowel changes had left indistinct from he (“he”) — by the 1400s, being readily incorporated alongside native words beginning with the same sound (the, that, this). Used as a singular pronoun since 1300, e.g. in the 1325 Cursor Mundi. The Norse term (whence also Icelandic þeir (“they”), Faroese teir (“they”), Danish de (“they”), Swedish de (“they”), Norwegian Nynorsk dei (“they”)) is from Proto-Germanic *þai (“those”) (from Proto-Indo-European *to- (“that”)), whence also Old English þā (“those”) (whence obsolete English tho), Scots thae, thai, thay (“they; those”). The origin of the determiner they (“the, those”) is unclear. The OED, English Dialect Dictionary and Middle English Dictionary define it and its Middle English predecessor thei as a demonstrative determiner or adjective meaning "those" or "the". This could be a continuation of the use of the English pronoun they's Old Norse etymon þeir as a demonstrative meaning "those", but the OED and EDD say it is limited to southern, especially southwestern, England, specifically outside the region of Norse contact.

pron

1. (the third-person plural nominative) A group of entities previously mentioned.
   Fred and Jane? They just arrived.
   Dogs may bark if they want to be fed.
   Plants wilt if they are not watered.
   I have a car and a truck, but they are both broken.
   […]purſued his vnneighbourly purpoſe in ſuch ſort: that hee being the ſtronger perſwader, and ſhe (belike) too credulous in beleeuing or elſe ouer-feeble in reſiſting, from priuate imparlance, they fell to action; and continued their cloſe fight a long while together, vnſeene and vvithout ſuſpition, no doubt to their equall ioy and contentment. 1620, Giovanni Bocaccio, translated by John Florio, The Decameron, Containing an Hundred Pleaſant Nouels: Wittily Diſcourſed, Betweene Seuen Honourable Ladies, and Three Noble Gentlemen, Isaac Iaggard, Nouell 8, The Eighth Day
   There is no reason to be scared of iguanas. They do not attack humans. 2010, Iguana Invasion!: Exotic Pets Gone Wild in Florida, page 9
2. (the third-person singular nominative, occasionally proscribed) A single person, previously mentioned, but typically not if previously named and identified as male or female, especially if of unknown or (since 21st century) non-binary gender.
   They requested a seat at Friday's performance but didn't say if they preferred the balcony or the floor.
   If someone enters the restricted area, they are required to present identification.
   Someone knocked into Harry as they hurried past him. It was Hermione. 1997, J. K. Rowling, chapter 10, in Harry Potter and the Philosopher's Stone, (quoted edition: London: Bloomsbury, 2000, page 187)
   One thing a nominee earns is the right to pick the vice president that they think will best reflect their vision of the country, and I am just glad I will have nothing to do with it. 2008, Michelle Obama, quoted in Lisa Rogak, Michelle Obama in Her Own Words, New York, NY: PublicAffairs, 2009. →ISBN, page 18
   The boycott, led by Elisha Lim, of a Toronto gay and lesbian newspaper after it refused to use their preferred pronoun ["they"], citing grammar considerations, inspired me. 2014, Ivan E. Coyote, Rae Spoon, Gender Failure
   I'm angry that we're here again, that we have had to tell another mother that their child is dead. 2015 April, Stephanie Rawlings-Blake (mayor of Baltimore), commenting on the death of Freddie Gray
3. (indefinite pronoun, vague meaning) People; some people; people in general; someone, excluding the speaker.
   They say it’s a good place to live.
   They didn’t have computers in the old days.
   They should increase our wages.
   Ha, you believe the moon is real? That's just what they want you to think.
   I am beautiful, no matter what they say 2002, Linda Perry (lyrics and music), “Beautiful”, in Stripped, performed by Christina Aguilera