> A reflexive pronoun is a pronoun that refers back to the subject of a sentence or clause. You can recognize a reflexive pronoun by its second half: they all end in -self or -selves. [...] Only the dual-purpose your* has two reflexive forms: singular yourself and plural yourselves. [...] But if you'd been typing in the late 1300s, themself would have been the default: it was the only version around until the mid-1400s. [...] Eventually, themselves became the only accepted form. But themself never fully disappeared.

https://www.merriam-webster.com/words-at-play/themself