that: [OE] That is a very ancient word, going right back to prehistoric Indo-European *tad (source also of Greek tó ‘the’ and Russian tot’, ta, to ‘that’). Its original function in English was as the neuter form of a demonstrative pronoun. This came to be used as the definite article (modern English the), but as the grammatical gender system began to break down in the early Middle English period, that broke away from the definite article and began to be used as a demonstrative adjective. Its use as a relative pronoun goes back to the Old English period. => the
