The Collaborative International Dictionary of English v.0.48:

Cognati \Cog*na"ti\, n. pl. [L.] (Law)
   Relatives by the mother's side. --Wharton.
   [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

COGNATI, cognates. This term occurs frequently in the Roman civil law, and
denotes collateral heirs through females. It is not used in the civil law as
it now prevails in France. In the common law it has no technical sense, but
as a word of discourse in English it signifies, generally, allied by blood,
related in origin, of the same family. See Vicat, ad verb.; also, Biret's
Vocabulaire.