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"strengthen" is also used as a crossword clue. If that's what you saw, the answer is often intensify (9 letters).
Definitions
give a healthy elasticity to
Word origin
From rare Middle English strengthenen (14th c.), from earlier strengthen (12th c.), where -en is the infinitive ending. Probably the original form was reinterpreted as strength + -en around the time when the infinitive ending was being apocopated in late Middle English.
Used in a sentence
“strengthen a muscle”
“strengthen a wall”
“strengthen one's willpower”
Source: Wiktionary, CC BY-SA 4.0
Used as a crossword answer1 curated clues
01“Fortify”10 letters
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