Etymology
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vulnerable (adj.)
c. 1600, from Late Latin vulnerabilis "wounding," from Latin vulnerare "to wound, hurt, injure, maim," from vulnus (genitive vulneris) "wound," perhaps related to vellere "pluck, to tear" (see svelte), or from PIE *wele-nes-, from *wele- (2) "to strike, wound" (see Valhalla).
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invulnerable (adj.)
1590s, from Latin invulnerabilis "invulnerable," from in- "not" (see in- (1)) + vulnerabilis (see vulnerable). Related: Invulnerably.
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off-base (adv.)

"unawares," by 1936, American English, from off (adv.) + base (n.); a figurative extension from baseball sense of a runner being "not in the right position" (1882) and vulnerable to being picked off.

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underbelly (n.)
c. 1600, from under + belly (n.). In figurative sense of "most vulnerable part" it is recorded from Churchill's 1942 speech. Sometimes used erroneously or euphemistically in sense of "seamy or sordid part" of anything.
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impregnable (adj.)
early 15c., imprenable "impossible to capture," from Old French imprenable "invulnerable," from assimilated form of in- "not, opposite of" (see in- (1)) + Old French prenable "assailable, vulnerable" (see pregnable). With restored -g- from 16c. Related: Impregnably.
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penetrable (adj.)

early 15c., "penetrating" (a sense now rare or obsolete), from Latin penetrabilis "penetrable, vulnerable," from penetrare (see penetrate). Meaning "capable of being entered or pierced by another body" is attested from 1530s; figurative use by 1590s. Related: Penetrability.

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Achilles tendon (n.)
from Modern Latin tendo Achillis, first used by German surgeon Lorenz Heister (1683-1758) and so-called in reference to the one vulnerable spot of the Greek hero Achilles, whose mother held him by the heel when she dipped him in the River Styx to render him invulnerable (this story is not in Homer and not found before 1c. C.E.). Earlier Achilles' sinew, from Modern Latin chorda Achillis, coined 1693 by Dutch anatomist Philip Verheyden when dissecting his own amputated leg. Hence figurative use of heel of Achillies for "vulnerable spot" (1810).
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set-up (n.)
"arrangement," 1890, from verbal phrase set up, attested from c. 1200 as "to make ready for use" and from 1950 (in pugilism) as "to bring (someone) to a vulnerable position;" from set (v.) + up (adv.). The verbal phrase also can mean "to establish" (early 15c.) and "put drinks before customers" (1880).
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pregnable (adj.)

of a fortress, etc., "capable of being taken or won by force," 1530s, an alteration of Middle English preignable, earlier prenable (early 15c.), pernable (late 14c.), from Old French prenable, pregnauble "assailable, vulnerable," from stem of prendre "to take, grasp, seize," from Latin prehendere "to take hold of, to seize" (from prae- "before," see pre-, + -hendere, from PIE root *ghend- "to seize, take"). The form was confused in French and English by the influence of unrelated words from French preignaunt and English pregnant.

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