"having a soft carapace" (of crabs, clams, tortoises), 1610s; see soft (adj.) + shell (n.).
Entries linking to soft-shelled
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.
shell n.
"hard outer covering," Middle English shel, shelle, from Old English sciell, scill, Anglian scell "seashell; eggshell," which is related to Old English scealu "shell, husk," from Proto-Germanic *skaljo "piece cut off; shell; scale" (source also of West Frisian skyl "peel, rind," Middle Low German schelle "pod, rind, egg shell," Gothic skalja "tile"), with the shared notion of "covering that splits off," from PIE root *skel- (1) "to cut." Italian scaglia "chip" is from Germanic.
Also in late Old English as "a coating or layer." The general sense of "protective outer covering of some invertebrates" is in Middle English (by c. 1400 as "house of a snail;" by 1540s in reference to a tortoise or turtle); the meaning "outer layer of a nut" (or a fruit considered as a nut) is by mid-14c. With notion of "mere exterior," hence "empty or hollow thing" by 1650s. The meaning "hollow framework" is from 1791; that of "structure for a band or orchestra" is attested from 1938. To be out of (one's) shell "emerged into life" is by 1550s.
Military use for "explosive projectile" is by 1640s, first of hand grenades, and originally in reference to the metal case in which the gunpowder and shot were mixed; the notion is of a "hollow object" filled with explosives. Hence shell shock, "traumatic reaction to the stress of battle," recorded by 1915.
Shell game "a swindle" is from 1890, from a version of the three-card game played with a pea and walnut shells.