Words related to the
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." As a conjunction ("Not that I loved Caesar less, but that I loved Rome more") it was originally the neuter pronoun or adjective that used practically as a definite article qualifying the whole sentence.
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.
in phrase for the nonce (Middle English for þe naness, c. 1200) "for a special occasion, for a particular purpose," a misdivision (see N for other examples) of for þan anes "for the once," in reference to a particular occasion or purpose, the þan being an altered form of the Middle English dative definite article þam (see the). The phrase was used from early 14c. as an empty filler in metrical composition.
As a conjunction, "in that case, therefore," in Old English. As an adjective, "being at that time," from 1580s. As a noun from early 14c. For further sense development, see than. Similar evolutions in other Germanic languages; Dutch uses dan in both senses, but German has dann (adv.) "then," denn (conj.) "than." Now and then "at various times" is attested from 1550s; earlier then and then (c. 1200).
Interjectional use is recorded from 1530s, used variously to emphasize certainty, encouragement, or consolation. To have been there "had previous experience of some activity" is recorded from 1877.