"She", for example, is younger than "he", and seems to be an amalgamation of two Old English female pronouns, Birkett says – "heo" and "seo". "[These] probably combined over time, to make 'she'," he says.

Another commonly used modern pronoun, "they" – along with "them" and "their" – is actually not Old English at all, according to Birkett. It arrived with Old Norse, a Scandinavian language spoken by the Vikings who invaded and settled in England from the 800s onwards. "They" then spread and replaced the Old English "hie".

The foreign "they" may have become popular for practical reasons, Birkett suggests: the native "hie" was potentially confusing as it could mean "they" but also, "her" – whereas "they" was distinct and therefore clearer.

Later, "they" was also occasionally used in the singular, as it is today when used as a gender-neutral pronoun, Birkett says. The singular "they" appears, for example, in the 14th-Century text "William and the Werewolf", as well as in "The Pardoner's Prologue", by Geoffrey Chaucer, written around the same time.

"Chaucer was using 'they' as a singular back in the 14h Century," says Birkett. "It's a very, very old usage, and very useful when you don't know the person, [and don't want to refer] to them as 'he' or 'she'."