they

First recorded in 1150–1200; Middle English thei, they, from Old Norse their “they”; replacing Old English hī(e); cognate with Old English thā, plural of thæt; see that

Traditional grammars have limited the use of they to refer only to a plural antecedent. These grammars recommended using the singular masculine he as if it were generic, referring to a man, woman, or humanity universally: Each person should bring his own lunch. Later, when generic he was criticized as sexist, he or she began to be used in its place. But in spite of its prescriptive prohibition, the pronoun they is also used to refer to a single person in three distinct ways.
The oldest type, and the one most accepted by style guides, is when they refers to a generic, unspecified individual, or to a person whose gender and other personal details are unknown or irrelevant. Generic and singular indefinite they and related case forms their and them are found in respected works, from Shakespeare’s The Comedy of Errors (There's not a man I meet but doth salute me/As if I were their well-acquainted friend) to Lewis Carroll’s Alice's Adventures in Wonderland (If everybody minded their own business … the world would go round a deal faster than it does), as well as in the writing of authors like Shelley, Swift, Scott, Dickens, and Austen.
Singular they is also used as a pronoun for a known, specified person, particularly when the individual is named with a job title or other noun phrase, instead of a proper name: My teacher had their car stolen. This specific singular they looks similar to the generic singular they, but is somewhat less acceptable in conservatively edited English.
Nonbinary singular use of they, their, and them has become widely accepted in the 21st century. It has become much more common for people in general to announce which pronoun they use or to ask what pronoun another person uses. Some people, especially nonbinary, genderqueer, and gender-nonconforming people, don't like being referred to with either the traditionally masculine he or the feminine she. In those cases, nonbinary singular they is used as one alternative: When Tyler was applying to college, they indicated their intended major on the application. In addition to he, she, and singular they, there are also gender-neutral neopronouns like ze used for this purpose. Singular they can also be used when a person’s pronouns aren’t known, or to avoid assigning gender, and some style guides have been recommending this usage since the mid-2010s.
And although they may be used as a singular pronoun, they still takes a plural verb, analogous to the use of "you are" to refer to one person: The student brought in a note to show why they were absent. See also he¹.

C12: thei from Old Norse their, masculine nominative plural, equivalent to Old English thā

It was formerly considered correct to use he, him, or his after pronouns such as everyone, no-one, anyone, or someone as in everyone did his best, but it is now more common to use they, them, or their, and this use has become acceptable in all but the most formal contexts: everyone did their best