Etymology
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aphasic (adj.)
"characterized by pathological loss of ability to speak," 1867, from aphasia + -ic. Aphasiac (1868) is better as the noun, "one suffering from aphasia," but both words have been used in both senses.
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self-confidence (n.)

"confidence of one's own judgment or ability, reliance on one's own powers without other aid," 1650s, a back-formation from self-confident, or else from self- + confidence.

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potency (n.)

"power, inherent strength, ability to accomplish or effect," mid-15c., potencie, from Latin potentia "power," from potentem "potent," from potis "powerful, able, capable," from PIE root *poti- "powerful; lord." 

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superpower (n.)
1944, in geopolitical sense of "nation with great interest and ability to exert force in worldwide theaters of conflict," from super- + power (n.). The word itself is attested in physical (electrical power) senses from 1922.
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humorist (n.)

1590s, "person with the ability to entertain by comical fancy, humorous talker or writer," also "person who acts according to his humors" (obsolete), from humor (n.) + -ist. Perhaps on model of French humoriste.

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click (n.)

"a small, sharp sound," 1610s, from click (v.). As a sound in certain South African languages, 1837. Click-beetle attested from 1830, so called from its ability, when on its back, to spring into the air with an audible click.

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diffidence (n.)
Origin and meaning of diffidence

c. 1400, "distrust, want of confidence, doubt of the ability or disposition of others," from Latin diffidentia "mistrust, distrust, want of confidence," from diffidere "to mistrust, lack confidence," from dis- "away" (see dis-) + fidere "to trust" (from PIE root *bheidh- "to trust, confide, persuade"). The opposite of confidence. Original sense (distrust of others) is obsolete; the modern sense is of "distrust of oneself, want of confidence in one's ability, worth, or fitness" (1650s), hence "retiring disposition, modest reserve."  

Diffidence is a defect: it is an undue distrust of self, with fear of being censured for failure, tending to unfit one for duty. [Century Dictionary]
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faculty (n.)
late 14c., "ability, opportunity, means, resources," from Old French faculte "skill, accomplishment, learning" (14c., Modern French faculté) and directly from Latin facultatem (nominative facultas) "power, ability, capability, opportunity; sufficient number, abundance, wealth," from *facli-tat-s, from facilis "easy to do," of persons, "pliant, courteous, yielding," from facere "to do" (from PIE root *dhe- "to set, put").

Academic sense "branch of knowledge" (late 14c.) was in Old French and probably was the earliest in English (it is attested in Anglo-Latin from late 12c.), on notion of "ability in knowledge" or "body of persons on whom are conferred specific professional powers." Originally each department was a faculty; the use in reference to the whole teaching staff of an entire college dates from 1767. Related: Facultative. The Latin words facultas and facilitas "were originally different forms of the same word; the latter, owing to its more obvious relationship to the adj., retained the primary sense of 'easiness', which the former had ceased to have before the classical period." [OED]
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discretion (n.)

c. 1300, dyscrecyounne, "ability to perceive and understand;" mid-14c., "moral discernment, ability to distinguish right from wrong;" c. 1400, "prudence, sagacity regarding one's conduct," from Old French discrecion and directly from Medieval Latin discretionem (nominative discretio) "discernment, power to make distinctions," in classical Latin "separation, distinction," noun of state from past-participle stem of discernere "to separate, distinguish" (see discern).

Phrase at (one's) discretion attested from 1570s (earlier in (one's) discretion, late 14c.), from sense of "power to decide or judge, power of acting according to one's own judgment" (late 14c.). The age of discretion (late 14c.) in English law was 14.

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-ile 
also -il, word-forming element denoting ability or capacity, from Old French -il or directly from Latin adjectival suffix -ilis. Used in classical and Medieval Latin to form ordinal numbers, which accounts for its use from late 19c. in statistics (percentile, etc.).
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