What is Affinitas Affinitatis?

Legal Definition
That connexion between two persons which has neither consanguinity nor affinity; as, the connexion between the husband's brother and the wife's sister. This connexion is formed not between the parties themselves, nor between one of spouses and the kinsmen of the other, but between the kinsmen of both. Ersk. Inst. B, 1, tit. 6, s. 8.
-- Bouviers Law Dictionary
Legal Definition
Remote relationship by marriage. That connection between parties arising from marriage which is neither consanfiinity nor affinity. Chinn v. State, 47 Ohio t. 575, 26 N. E. 986, 11 In R. A. 630.
-- Black's Law Dictionary