to increase the ability of (as a muscle) to exert physical force <weight lifting will help toughen those flabby muscles of yours>
to make able to withstand physical hardship, strain, or exposure <hiking every morning through snow and ice toughened him considerably> Related Words acclimate
; break in
to make more harsh, uncompromising, or severe <attempts to toughen conditions under which waivers from military service would be permitted>
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