Etymology
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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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strengthen (v.)
late 14c., from strength + -en (1). Related: Strengthened; strengthening; strengthener. Earlier verb was simply strength (12c.).
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wrath (n.)
Old English wræððu "anger," from wrað "angry" (see wroth) + -þu, from Proto-Germanic -itho (as in strength, width etc.; see -th (2)).
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-th (2)
suffix forming nouns of action, state, or quality from verbs or adjectives (such as depth, strength, truth), from Old English -ðu, , from Proto-Germanic *-itho (cognates: Old Norse , Old High German -ida, Gothic -iþa), abstract noun suffix, from PIE *-ita (cognates: Sanskrit -tati-; Greek -tet-; Latin -tati-, as in libertatem "liberty" from liber "free"). Sometimes in English reduced to -t, especially after -h- (as in height).
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fortitude (n.)

late 14c., "moral strength (as a cardinal virtue); courage," from Latin fortitudo "strength, force, firmness, manliness," from fortis "strong, brave" (see fort). From early 15c. as "physical strength."

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asthenia (n.)
"weakness, debility," 1788, medical Latin, from Greek astheneia "want of strength, weakness, feebleness, sickness; a sickness, a disease," from asthenes "weak, without strength, feeble," from a- "not, without" (see a- (3)) + sthenos "strength, power, ability, might," which is of uncertain origin, perhaps from PIE root *segh- "to have, hold," on the notion of "steadfastness, toughness."
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force majeure (n.)
1883, French, "superior strength."
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ammeter (n.)
instrument for measuring the strength of electric currents, 1882, from ampere + -meter.
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Hebe (1)

c. 1600, Greek goddess of youth, daughter of Zeus and Hera, wife of Hercules, from Greek hēbē "youth, youthful prime, strength of youth" (legally, "the time before manhood," in Athens 16, in Sparta 18), from PIE *yeg-wa- "power, youth, strength."

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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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