expertness

From expert +‎ -ness.

expertness (usually uncountable, plural expertnesses)

1. The state of being expert.
2. Proficiency or skill.
   • 1861, E. J. Guerin, Mountain Charley, page 22:

     I had […] armed myself with a revolver, in the use of which I finally reached a certain degree of expertness from frequent practice.

   • 1902, William James, The Varieties of Religious Experience: A Study in Human Nature […] , New York, N.Y.; London: Longmans, Green, and Co. […], →OCLC, page 347:

     Not only must she receive unheard-of personal favors and spiritual graces from her Saviour, but she must immediately write about them and exploiter them professionally, and use her expertness to give instruction to those less privileged.