"potash soap, liquid soap made with potash as a base," early 15c., from soft (adj.) + soap (n.). So called because it does not harden into cakes. The figurative sense of "flattery" is recorded from 1830.
Entries linking to soft-soap
soft adj.
Old English softe, earlier sefte, "gentle, mild-natured; easeful, comfortable, calm, undisturbed; luxurious," from West Germanic *samfti, from Proto-Germanic *samftijaz "level, even, smooth, gentle, soft" (source also of Old Saxon safti, Old High German semfti, German sanft; and from a variant form with -ch- for -f-, Middle Dutch sachte, Dutch zacht, German sacht), from PIE root *sem- (1) "one; as one, together with" (the source of seem andsame). The etymological sense would be "smooth and agreeable," to the touch, senses, or mind.
From c. 1200 of persons, hearts, etc., "tender, yielding to emotions," also "easily moved or swayed; soft-hearted, sympathetic; docile." It is attested from late 14c. as "indulgent," also "physically feeble; effeminate, easily overcome, lacking manly courage." The meaning "foolish, simple, silly" is attested from 1620s.
It is attested from mid-13c. of material things, "not stiff, not coarse, fine; yielding readily to pressure." Of sounds or voices, "quiet, not loud or harsh," from early 13c. Of words, "mild, restrained; courteous" mid-14c. From late 14c. of wind, rain, etc.
From 1755 of water ("relatively free from mineral salts"), from 1789 of coal (generally meaning bituminous as opposed to anthractic). In reference to drinks, "non-alcoholic" by 1880. Of letters (-c-, -g-, etc.) when pronounced with more sibilance and less plosiveness, 1630s.
In reference to technology, "using natural resources," by 1974, perhaps 1970. Of a science or methods or data, "not subject to experimental verification, not mathematical," c. 1960.
Many phrases simply are contrasts to earlier ones in hard: Soft landing is from 1958 and the U.S. space program; soft rock as a music style is attested from 1969. Soft spot "weak or vulnerable place," literal and figurative, is by 1933, colloquial. The soft sell sales pitch that relies on gentle persuasions is so called by 1955. Soft-shoe as a dancing style is attested from 1927. The photographic soft-focus (adj.), in reference to camera lenses or shots, is from 1917; figurative use of it is by 1961. The softer sex "women collectively" is from 1640s.
soap n.
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.