/ðeɪ/
[pronoun] A word used to refer to two or more people or things already mentioned, or to a single person when their gender is not known or not important.
'They' is not English at all — it was borrowed from Viking invaders because English speakers found their own word too confusing.
Few words in everyday English carry as dramatic a backstory as 'they.' Before the Vikings arrived in Britain, Old English speakers used 'hīe' or 'hī' as their third-person plural pronoun — the word that meant 'those people over there.' But 'hīe' had an inconvenient problem: it sounded almost identical to the Old English word for 'he,' which was 'hē.' In the busy, noisy world of spoken communication, this confusion was more than a minor irritant. Entire conversations could tip into misunderstanding over whether a speaker meant one man or a whole crowd of people.
The solution arrived not from within English itself but from the language of the Norse settlers who flooded into northern and eastern England from the 8th century onward. These Scandinavian newcomers spoke Old Norse, and in Old Norse the plural pronoun was 'þeir,' which was clear, distinct, and unmistakably plural. The symbol 'þ,' called a thorn, represents the 'th' sound we still use today. As English and Old Norse speakers lived side by side — trading, marrying, farming together — words began to flow between the two languages, and 'þeir' gradually slipped into northern English dialects first, before spreading southward across the whole country. The chain of transmission runs: Old Norse 'þeir' → northern Middle English 'þei' → Middle English 'they' → Modern English 'they.'
This borrowing is part of one of the most remarkable episodes in the history of any language. English did not merely borrow a handful of Norse nouns the way it later borrowed French cooking terms or Italian music words. It borrowed the very skeleton of its grammar — core pronouns that speakers use dozens of times every day. Alongside 'they,' the Norse also gave English 'them' (from Old Norse 'þeim') and 'their' (from Old Norse 'þeira'), replacing the Old English forms 'heom' and 'hiera' almost completely. The entire third-person plural pronoun system in English is Scandinavian, not Anglo-Saxon.
By the late 13th and early 14th centuries, 'they' was appearing in major English texts, including the works associated with the period of Middle English literature. Geoffrey Chaucer used it comfortably in the 14th century, and by the time William Shakespeare was writing in the late 16th century, 'they' was entirely standard — no one thought of it as a foreign word any longer. It had become more English than English itself.
What makes 'they' especially alive as a topic today is its long history as a singular pronoun. Many people assume that using 'they' to refer to one person is a modern invention, but this use stretches back at least to the 14th century. Writers from Chaucer to Shakespeare to Jane Austen used singular 'they' naturally when the gender of a person was unknown or unimportant — phrases like 'everyone should do their best' appear throughout classic English literature. The singular 'they' is, in fact, one of the oldest tricks in the English language's bag, used for centuries before anyone thought to question it.