Strength

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Old English strengthu, from the Germanic base of strong.

wiktionary

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From Middle English strengthe, from Old English strengþu(“strength”), from Proto-West Germanic *strangiþu(“ strongness; strength”), equivalent to strong +‎ -th. Cognate with Dutch strengte(“strength”), German Low German Strengde, Strengte(“ harshness; rigidity; strictness; severity”).

etymonline

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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,