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" (perhaps also the 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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bath (n.)

Old English bæð "an immersing of the body in water, mud, etc.," also "a quantity of water, etc., for bathing," from Proto-Germanic *badan (source also of Old Frisian beth, Old Saxon bath, Old Norse bað, Middle Dutch bat, German Bad), from PIE root *bhē- "to warm" + *-thuz, Germanic suffix indicating "act, process, condition" (as in birth, death). The etymological sense is of heating, not immersing.

The city in Somerset, England (Old English Baðun) was so called from its hot springs. Bath salts is attested from 1875 (Dr. Julius Braun, "On the Curative Effects of Baths and Waters"). Bath-house is from 1705; bath-towel is from 1958.

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soap (v.)

1580s, from soap (n.). Related: Soaped; soaping.

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sitz-bath (n.)

"hip-bath," also a tub adapted for such a bath, 1849, a hybrid from German Sitzbad, literally "bath in a sitting position," from German sitzen (see sit (v.)) with English bath for cognate German Bad.

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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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soft-soap (n.)

1630s, from soft (adj.) + soap (n.). Figurative sense "flattery" is recorded from 1830.

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soap opera (n.)

"melodramatic radio serial" (later extended to television), 1939; so-called because sponsors often were soap manufacturers, from earlier horse opera "a Western" (1927). Shortened form soap for this first attested 1943.

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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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bird-bath (n.)

also birdbath, "small basin filled with water, placed in a garden, etc. for wild birds to drink from and bathe in," 1862, from bird (n.1) + bath (n.).

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