Etymology
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vigor (n.)
c. 1300, "physical strength, energy in an activity," from Anglo-French vigour, Old French vigor "force, strength" (Modern French vigueur), from Latin vigorem (nominative vigor) "liveliness, activity, force," from vigere "be lively, flourish, thrive," from PIE root *weg- "to be strong, be lively."
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vigour (n.)
chiefly British English spelling of vigor (q.v.); for spelling, see -or.
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invigorate (v.)
1640s, from in- (2) + vigor (n.) + -ate (2). Earlier verb was envigor (1610s), from Old French envigorer. Related: Invigorated; invigorating.
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vigorous (adj.)
c. 1300 (early 13c. as a surname), from Anglo-French vigrus, Old French vigoros "strong, robust, powerful" (12c., Modern French vigoreux), from Medieval Latin vigorosus, from Latin vigere "be lively, flourish, thrive" (see vigor). Related: Vigorously.
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*weg- 
Proto-Indo-European root meaning "to be strong, be lively."

It forms all or part of: awake; bewitch; bivouac; invigilate; reveille; surveillance; vedette; vegetable; velocity; vigil; vigilant; vigilante; vigor; waft; wait; wake (v.) "emerge or arise from sleep;" waken; watch; Wicca; wicked; witch.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit vajah "force, strength," vajayati "drives on;" Latin vigil "watchful, awake," vigere "be lively, thrive," velox "fast, lively," vegere "to enliven," vigor "liveliness, activity;" Old English wacan "to become awake," German wachen "to be awake," Gothic wakan "to watch."
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punchy (adj.2)
"full of vigor," 1926, from punch (n.3) + -y (2). Related: Punchily; punchiness.
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pep (n.)

"vigor, energy," 1912, shortened form of pepper (n.), which was used in the figurative sense of "spirit, energy" from at least 1847. Pep rally "meeting to inspire enthusiasm" is attested from 1915; pep talk is from 1926. To pep (something) up "fill or inspire with vigor or energy" is from 1925.

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lustless (adj.)
early 14c., "wanting vigor or energy," from lust (n.) + -less. From 1580s as "wanting sexual appetite."
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dull (v.)

c. 1200, "to lessen the vigor, activity, or sensitiveness of" (transitive), from dull (adj.). Of pointed or edged things, "make less sharp, render blunt," from late 14c. Of colors, glass, etc., "remove the brightness or clearness of," late 14c. Intransitive sense of "lose vigor, intensity, or keenness" is from late 14c. Related: Dulled; dulling.

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nerve (v.)
c. 1500, "to ornament with threads;" see nerve (n.). Meaning "to give strength or vigor" is from 1749. Related: Nerved; nerving.
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