Etymology
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sweet (adj.)

Old English swete "pleasing to the senses, mind or feelings; having a pleasant disposition," from Proto-Germanic *swotja- (source also of Old Saxon swoti, Old Frisian swet, Swedish söt, Danish sød, Middle Dutch soete, Dutch zoet, Old High German swuozi, German süß), from PIE root *swād- "sweet, pleasant" (Sanskrit svadus "sweet;" Greek hedys "sweet, pleasant, agreeable," hedone "pleasure;" Latin suavis "pleasant" (not especially of taste), suadere "to advise," properly "to make something pleasant to"). Words for "sweet" in Indo-European languages typically are used for other sense as well and in general for "pleasing."

Then come kiss me, sweet-and-twenty!
Youth's a stuff will not endure.
["Twelfth Night"]

Also "being in a sound or wholesome state" (mid-13c.), and, of water, "fresh, not salt" (late Old English). As an intensifier from 1958. Sweet in bed (c. 1300) was the equivalent of modern "good in bed." To be sweet on someone is first recorded 1690s. Sweet sixteen first recorded 1767. Sweet dreams as a parting to one going to sleep is attested from 1897, short for sweet dreams to you, etc. Sweet-and-sour in cookery is from 1723 and not originally of oriental food. Sweet nothings "sentimental trivialities" is from 1900. Sweet spot is from 1976, first in reference to tennis rackets. Sweet corn is from 1640s.

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sweet (n.)
c. 1300, "something sweet to the taste," also "beloved one," from sweet (adj.); the specific meaning "candy drop" is 1851 (earlier sweetie, 1721). Meaning "one who is dear to another" is from 14c. Old English swete (n.) meant "sweetness."
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sweet-briar (n.)
"eglantine," 1530s, from sweet (adj.) + briar (n.).
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sweet tooth (n.)
"fondness for sugary stuff," late 14c., from sweet (adj.) + tooth in the sense of "taste, liking" (see toothsome).
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sweet-grass (n.)
1570s, from sweet (adj.) + grass (n.). Perhaps so called for the fondness of cattle for it.
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sweet-talk (v.)

Sweet-talk, 1935, from noun phrase; see sweet (adj.) + talk (n.). Earliest usages seem to refer to conversation between black and white in segregated U.S.

"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]

Latin had suaviloquens, literally "sweet-spoken."

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sweetie (n.)
1721, "lollipop;" 1778, "lover, sweetheart," from sweet (n.) + -ie.
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sweetness (n.)
Old English swetnes; see sweet (adj.) + -ness.
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sweetly (adv.)
Old English swetlice; see sweet (adj.) + -ly (2).
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