Etymology
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weak (adj.)

c. 1300, from Old Norse veikr "weak," cognate with Old English wac "weak, pliant, soft," from Proto-Germanic *waika- "yield" (source also of Old Saxon wek, Swedish vek, Middle Dutch weec, Dutch week "weak, soft, tender," Old High German weih "yielding, soft," German weich "soft"), from PIE root *weik- (2) "to bend, to wind."

Sense of "lacking authority" is first recorded early 15c.; that of "lacking moral strength" late 14c. In grammar, denoting a verb inflected by regular syllabic addition rather than by change of the radical vowel, from 1833. Related: Weakly. Weak-kneed "wanting in resolve" is by 1856; older in a literal sense.

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weakfish (n.)
1838, from Dutch weekvisch, from week "soft" (see weak). So called because it does not pull hard when hooked.
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weaken (v.)
late 14c., "to become feeble," from weak + -en (1). Transitive sense from 1560s. Related: Weakened; weakening.
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weakness (n.)
c. 1300, "quality of being weak," from weak + -ness. Meaning "a disadvantage, vulnerability" is from 1590s. That of "self-indulgent fondness" is from 1712; meaning "thing for which one has an indulgent fondness" is from 1822.
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weakling (n.)
1520s, coined by Tyndale from weak (adj.) + -ling as a loan-translation of Luther's Weichling "effeminate man" (from German weich "soft") in I Corinthians vi.9, where the Greek is malakoi, from malakos "soft, soft to the touch," "Like the Lat. mollis, metaph. and in a bad sense: effeminate, of a catamite, a male who submits his body to unnatural lewdness" ["Thayer's Greek-English Lexicon of the New Testament"].
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*weik- (2)

also *weig-, Proto-Indo-European root meaning "to bend, to wind."

It forms all or part of: vetch; vicar; vicarious; vice- "deputy, assistant, substitute;" viceregent; vice versa; vicissitude; weak; weakfish; week; wicker; wicket; witch hazel; wych.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit visti "changing, changeable;" Old English wac "weak, pliant, soft," wician "to give way, yield," wice "wych elm," Old Norse vikja "to bend, turn," Swedish viker "willow twig, wand," German wechsel "change."

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asthenic (adj.)
"characterized by debility, weak," 1788, from Latinized form of Greek asthenikos, asthenes "weak, without strength, feeble" (see asthenia).
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foible (n.)
1640s, "weak point of a sword blade" (contrasted to forte), from French foible "a weak point, a weakness, failing," from noun use of Old French adjective feble "feeble" (see feeble). The spelling borrowed in English is obsolete in modern French, which uses faible. Extended sense of "weak point of character" is first recorded 1670s. Related: Foibles.
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debilitative (adj.)

"tending to render weak or infirm," 1680s, with -ive + Latin debilitat-, stem of debilis "lame, disabled, crippled," figuratively "weak, helpless," from de "from, away" (see de-) + -bilis "strength," from PIE root *bel-"strong" (see Bolshevik). 

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imbecile (adj.)

1540s, imbecille "weak, feeble" (especially in reference to the body), from French imbecile "weak, feeble" (15c.), from Latin imbecillus "weak, feeble," a word of uncertain origin.

The Latin word traditionally is said to mean "unsupported" or "without a walking stick" (Juvenal: imbecillis: quasi sine baculo), from assimilated form of in- "not, opposite of" + baculum "a stick" (see bacillus), but Century Dictionary finds that "improbable" and de Vaan finds it "very far-fetched" and adds "it seems to me that exactly the persons who can walk without a support are the stronger ones." There also are phonological objections.

Sense shifted to "mentally weak or incapable" from mid-18c. (compare frail, which in provincial English also could mean "mentally weak"). As a noun, "feeble-minded person," it is attested from 1802. Traditionally an adult with a mental age of roughly 6 to 9 (above an idiot but beneath a moron).

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