fwiw, OED2:

1. Descended from a common ancestor; of the same stock or family.
1827 G. Higgins Celtic Druids 78 Some of their cognate tribes. 1864 Kirk Chas. Bold II. iv. ii. 251 The barriers between cognate states. 1880 Muirhead tr. Instit. Gaius i. §156 Agnates are+persons who are of kin through males,—cognate, as it were, through the father.

2. Of languages: Descended from the same original language; of the same linguistic family. Of words: Coming naturally from the same root, or representing the same original word, with differences due to subsequent separate phonetic development; thus, Eng. five, L. quinque, Gr. pŒmse, are cognate words, representing a primitive *pénke.