Etymology
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audacity (n.)
early 15c., "boldness, courage, daring; vigor, animation," from Medieval Latin audacitas "boldness," from Latin audacis genitive of audax "bold, daring; rash, foolhardy" (see audacious). In English, the meaning "presumptuous impudence," implying contempt of moral restraint, is from 1530s.
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vivacity (n.)
early 15c., "liveliness, vigor," from Old French vivacite or directly from Latin vivacitatem (nominative vivacitas) "vital force, liveliness," from vivax (genitive vivacis) "lively," also "long-lived," from vivere "to live" (see vital).
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fade (n.)
early 14c., "loss of freshness or vigor," from fade (adj.), c. 1300, " lacking in brilliance; pale, discolored, dull," from Old French fade (see fade (v.)). As a type of tapering hairstyle from 1988 (fade-out style is in a 1985 "Ebony" article on men's haircuts).
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faintly (adv.)
c. 1300, "dispiritedly, timidly, half-heartedly;" early 14c. "feebly, wearily, without vigor;" from faint (adj.) + -ly (2). Meaning "indistinctly" is from 1580s. Also in Middle English, "deceitfully, hypocritically, falsely" (mid-14c.).
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lusty (adj.)
early 13c., "joyful, merry;" late 14c., "full of healthy vigor," from lust (n.) + -y (2). Used of handsome dress, fine weather, good food, pleasing language, it largely escaped the Christianization and denigration of the noun in English. The sense of "full of desire" is attested from c. 1400 but seems to have remained secondary. Related: Lustily; lustiness.
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pike (n.3)

"type of long, slender, voracious freshwater fish," early 14c., pik (mid-12c. in place names), probably short for pike-fish, a special use of pike (n.2) in reference to the fish's long, pointed jaw, and in part from French brochet "pike" (fish), from broche "a roasting spit." In Middle English, proverbial for health and vigor.

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verve (n.)
1690s, "special talent in writing, enthusiasm in what pertains to art and literature," from French verve "enthusiasm" (especially pertaining to the arts), in Old French "caprice, odd humor, proverb, saying; messenger's report" (12c.), probably from Gallo-Roman *verva, from Latin verba "(whimsical) words," plural of verbum "word" (see verb). Meaning "mental vigor" is first recorded 1803.
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debility (n.)

"state or condition of being weak or feeble, lack of strength or vigor," early 15c., from Old French debilite (Modern French débilité) or directly from Latin debilitatem (nominative debilitas) "a laming, crippling, weakening," from 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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brio (n.)

"liveliness, vivacity," 1734, from Italian brio "mettle, fire, life," perhaps a shortened derivative of Latin ebrius "drunk." Or via Provençal briu "vigor," from Celtic *brig-o- "strength," from PIE root *gwere- (1) "heavy." Probably it entered English via the musical instruction con brio.

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strength (n.)
Old English strengþu, strengð "bodily power, force, vigor, firmness, fortitude, manhood, violence, moral resistance," from Proto-Germanic *strangitho (source also of Old High German strengida "strength"), from PIE *strenk- "tight, narrow" (see string (n.)), with Proto-Germanic abstract noun suffix *-itho (see -th (2)). Compare length/long. From the same root as strong,
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