Definition of "inveterateness"

Twas the first play he praised on mortality; and as time hath rendred him more perfect in the Art, so hath the inveterateness of his malice more ready in the execution

1650, Thomas Browne, Pseudodoxia Epidemica: […], 2nd edition, London: […] A[braham] Miller, for Edw[ard] Dod and Nath[aniel] Ekins, […]