Etymology
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soap (n.)

Old English sape "soap, salve" (originally a reddish hair dye used by Germanic warriors to give a frightening appearance), from Proto-Germanic *saipon "dripping thing, resin" (source also of Middle Low German sepe, West Frisian sjippe, Dutch zeep, Old High German seiffa, German seife "soap," Old High German seifar "foam," Old English sipian "to drip"), from PIE *soi-bon-, from root *seib- "to pour out, drip, trickle" (source also of Latin sebum "tallow, suet, grease").

Romans and Greeks used oil to clean skin; the Romance words for "soap" (Italian sapone, French savon, Spanish jabon) are from Late Latin sapo "pomade for coloring the hair" (first mentioned in Pliny), which is a Germanic loan-word, as is Finnish saippua. The meaning "flattery" is recorded from 1853.

Soap opera is recorded from 1939, as a disparaging reference to daytime radio dramas sponsored by soap manufacturers. 

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soap (v.)
1580s, from soap (n.). Related: Soaped; soaping.
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soft-soap (n.)
1630s, from soft (adj.) + soap (n.). Figurative sense "flattery" is recorded from 1830.
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soap-dish (n.)
1835 as a dish for a bar of soap; 1814 as a holder for shaving-soap, from soap (n.) + dish (n.).
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soap-box (n.)
also soapbox, 1650s, "box for holding soap," later especially a wooden crate in which soap may be packed; from soap (n.) + box (n.). Typical of a makeshift stand for a public orator at least since 1907. Also used by children to make racing carts, as in soap-box derby, annual race in Dayton, Ohio, which dates to 1933.
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soapstone (n.)
type of talc, 1680s, from soap (n.) + stone (n.). So called because it is occasionally used for cleaning.
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soapy (adj.)
c. 1600, from soap (n.) + -y (2). Related: Soapily; soapiness.
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sebum (n.)
secretion of the sebaceous glands, 1728, from medical use of Latin sebum "sebum, suet, grease," probably related to sapo "soap" (see soap (n.)).
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saponification (n.)
1801, from French saponification, from saponifier, from Modern Latin saponificare, from sapon- "soap" (see soap (n.)) + -ficare, combining form of Latin facere "to make" (from PIE root *dhe- "to set, put").
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