Definition of firm up phrasal verb from the Oxford Advanced Learner's Dictionary

 

firm up

 phrasal verb
phrasal verb
 
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Phrasal Verbs
to become harder or more solid Put the mixture somewhere cool to firm up.
  1. 1to make arrangements more final and fixed The company has not yet firmed up its plans for expansion. The precise details still have to be firmed up.
  2. 2to make something harder or more solid A few weeks of aerobics will firm up that flabby stomach.
See the Oxford Advanced American Dictionary entry: firm up