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. Later, when generic he was criticized as sexist, the long and awkward he or she began to be used in its place. But in spite of prescriptive rules that would prohibit it, the pronoun they is also used to refer to a single person in three distinct ways, and each of these three uses grew out of a particular historical or social context.
Shakespeare, Swift, Shelley, Scott, and Dickens, as well as many other English and American writers, used they to refer to a generic, unspecified individual, or to a person whose gender and other personal details were unknown or irrelevant. So this use of they, their, and them is not a recent development, nor is it a mark of ignorance. 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).
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. The third person singular pronouns in English are traditionally binary, with the masculine he and the feminine she. People, including many who are nonbinary and gender-nonconforming, have simply chosen between these two words. Likewise, people apply these pronouns to others based on gender expression cues observed in their appearance. By the mid-2010s, some style guides began recommending the use of singular they as one way to refer to an individual without assigning gender to that person. At the same time, it has become much more common for people to announce their pronoun or ask what pronoun a person uses. It may be that a person has chosen a traditional binary pronoun like he or she, an alternative gender-neutral or nonbinary pronoun such as ze, or the singular use of the existing pronoun they: When Tyler was applying to college, they indicated their intended major on the application. In spite of the older grammar rules that prohibited the use of singular they in reference to a specified, known, or named person, use of they when the antecedent is a gender-nonconforming individual or one who does not identify as male or female is now accepted as an option.
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.