Definition of "firm up"

firm up

verb
third-person singular simple present firms up, present participle firming up, simple past and past participle firmed up
1. (transitive, idiomatic) To make (tentative plans) more definite.
  Quotations
  Just got a call from Falces office firming up my hospital date.
  1977-1980, Lou Sullivan, personal diary, quoted in 2019, Ellis Martin, Zach Ozma (editors), We Both Laughed In Pleasure
  Sometime soon (if it isn't happening already), Labour leader Sir Kier Starmer and his team will be firming up plans for what they want to do in the crucial first 100 days of a new government.
  2023 September 6, Nigel Harris, “Comment: Leaderless and rudderless”, in RAIL, number 991, page 3
2. (intransitive, idiomatic, of tentative plans) To become more definite.
3. (transitive) To make (muscles) more toned through physical exercise.
4. (intransitive, of muscles) To become more toned through physical exercise.
5. (intransitive) To become firmer in consistency.