"worthless person," 1946, British slang, a southern variant of Scottish get "illegitimate child, brat," which is attested by 1706 ("Gregor Burgess protested against the said Allane that called him a witch gyt or bratt"), according to "Dictionaries of the Scots Languages"); related to beget on the notion of "what is got." Scots get, gyt, geitt, etc. also can be an affectionate term for a child. 

1711, "the real point" (of a law case, etc.), from Anglo-French legalese phrases such as cest action gist "this action lies," from Old French gist en "it consists in, it lies in," from gist (Modern French gît), third person singular present indicative of gésir "to lie," from Latin iacet "it lies," from iacēre "to lie, rest," related to iacere "to throw" (from PIE root *ye- "to throw, impel"). Extended sense of "essence" first recorded 1823.

1935, American English, from noun phrase; see sweet (adj.) + talk (n.). Earliest print uses are in a racial context.

"I ain' gonna stay heah no longah. Don' nevah keer, ef I do git cotched—or die. Tha's bettah than to stay heah an' listen to Maw Haney sweet-talk the white folks, whilst they drives us clean to the grave. ..." [The Crisis, July 1935]

Sweet-mouth (v.) "flatter" is noted in 1948 as a Gullah term. Latin had suaviloquens, literally "sweet-spoken."

Old English mast "greatest in number, amount, or extent; largest," earlier mæst, from Proto-Germanic *maistaz (source also of Old Saxon mest, Old Frisian mast, Old Norse mestr, Dutch meest, German meist, Gothic maists "most"), superlative form of Proto-Germanic *maiz, root of Old English ma, mara (see more). Used in Old English as superlative of micel "great, large" (see mickle), hence, in later use, superlative of much. The vowel has been influenced by more.

Original sense of "greatest" survives in phrase for the most part (mid-14c.; late Old English had þa mæste dæl). Slang the most meaning "the best, extremely good" is attested from 1953. Also used as an adverb in Old English and in late Old English as a noun, "the greatest or greater number." The sense of "greatest value or advantage" in the phrase make the most of (something) is by 1520s. Related: Mostly.

Double superlative mostest "greatest amount or degree" is by 1849 in U.S. Southern and African-American vernacular. The formula for victory in battle attributed to famously unschooled Confederate Lt. Gen. Nathan Bedford Forrest is first attested (1886) as Git thar the fastest with the mostest men.

From 15c.-17c. English also had mostwhat "for the most part," mostwhen "on most occasions," mostwhere "in most places."

"gypsy," 1834, from Spanish Gitano (fem. Gitana), from Vulgar Latin *Ægyptanus "Egyptian" (see Gypsy). The fem. is gitana. The French form of the feminine, gitane, was used as the name of a brand of cigarettes (1933) and has come to be used for French cigarettes generally.

old wire-strung instrument like a guitar, late 14c., from Old French guiterne, obscurely from Latin cithara, from Greek kithara (see guitar).