Expertness, n.
Skill derived from practice; readiness; as, expertness in seamanship, or in reasoning.
Syn. -- Facility; readiness; dexterity; adroitness; skill. See Facility.
Shades of meaning
Fernald has no entry of its own for expertness; he lists it under dexterity, ease, power.
Where he writes about it:
Expertness applies to the more mechanical processes of body and mind; we speak of the readiness of an orator, but of the expertness of a gymnast. — from ease