Middle English sope, from Old English sape "soap, salve," anciently 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 cleanse the 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 figurative meaning "flattery" is recorded from 1853.
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, the annual race in Dayton, Ohio, which dates to 1933.
"a secretion of the sebaceous glands," 1728, from medical use of Latin sebum "sebum, suet, grease," which is perhaps related to sapo "soap" (see soap (n.)), but de Vaan is skeptical and gives it no etymology.