Origin and history of tho

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 originally was the neuter pronoun or adjective that used practically as a definite article qualifying the whole sentence.

Slang that way "in love" is attested by 1929 (also, by 1960, "homosexual"). That-a-way "in that direction" is recorded from 1839. "Take that!" said while delivering a blow, is recorded from early 15c. That is, for "that is to say," is by late 12c. That's what "just so" is by 1790. The intensifier at that "as well, to boot" is by 1830, U.S. colloquial, perhaps from "(cheap) at that (price)," etc.

c. 1200, "notwithstanding that, conceding or allowing that," introducing a clause, used in contrasting or in correlation, from Old English þeah, þeh "though, although, even if, however, nevertheless, still, yet;" and in part from Old Norse þo "though," both from Proto-Germanic *thaukh (source also of Gothic þauh, Old Frisian thach, Middle Dutch, Dutch doch, Old High German doh, German doch), from PIE demonstrative pronoun *to- (see that).

The evolution of the terminal sound did not follow laugh, tough, etc., though a tendency to end the word in "f" existed c. 1300-1750 and persists in dialects.