thatyoudaoicibaDictYouDict[that 词源字典]
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[that etymology, that origin, 英语词源]
that (pron.)youdaoicibaDictYouDict
Old English þæt, "that, so that, after that," neuter singular demonstrative pronoun ("A Man's a Man for a' that"), relative pronoun ("O thou that hearest prayer"), and demonstrative adjective ("Look at that caveman go!"), corresponding to masc. se, fem. seo. From Proto-Germanic *that, from PIE *tod-, extended form of demonstrative pronominal base *-to- (see -th (1)). With the breakdown of the grammatical gender system, it came to be used in Middle English and Modern English for all genders. Germanic cognates include Old Saxon that, Old Frisian thet, Middle Dutch, Dutch dat "that," German der, die, das "the."

Generally more specific or emphatic than the, but in some cases they are interchangeable. From c. 1200 opposed to this as indicating something farther off. In adverbial use ("I'm that old"), in reference to something implied or previously said, c. 1200, an abbreviation of the notion of "to that extent," "to that degree." Slang that way "in love" first recorded 1929. That-a-way "in that direction" is recorded from 1839. "Take that!" said while delivering a blow, is recorded from early 15c.