Etymology
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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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Definitions of strengthen

strengthen (v.)
make strong or stronger;
This exercise will strengthen your upper body
strengthen the relations between the two countries
Synonyms: beef up / fortify
strengthen (v.)
gain strength;
His body strengthened
strengthen (v.)
give a healthy elasticity to;
Synonyms: tone / tone up
From wordnet.princeton.edu