different between keenness vs ardour

English

keen +? -ness

keenness (countable and uncountable, plural keennesses)

ardour (countable and uncountable, plural ardours)

Britain, Canada, and Australia spelling of ardor
- 1931, H. P. Lovecraft, The Whisperer in Darkness, chapter 6:
  The purpose of my visit, and the frightful abnormalities it postulated struck at me all at once with a chill sensation that nearly over-balanced my ardour for strange delvings.

ardour f (oblique plural ardours, nominative singular ardour, nominative plural ardours)

Late Anglo-Norman spelling of ardur
toun ardour et l’estudie de aprendre […] deit estre provee

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