Singular they - Wikipedia
https://en.wikipedia.org/wiki/Singular_theyLike the "singular you", "singular they" permits a singular antecedent, but is used with the same verb forms as plural they, and has the same inflected forms as plural they (i.e. them, their, and theirs), except that in the reflexive form, themself is sometimes used instead of themselves. Themself is attested from the 14th to 16th centuries. Its use has been increasing since the 1970s or 1980s, though it is sometimes still classified as "a minority form". In 2002, Payne and Huddles…