μεσιτεύω (< μεσίτης), to interpose, mediate: ὅρκῳ, Heb.6:17 (MM, ii, 111).† (AS)
Thayer:
1) to act as a mediator between litigating or covenanting parties 1a) to accomplish something by interposing between two parties 1b) to mediate 2) act as a sponsor or surety 2a) to pledge one's self, give surety
μεσιτεύω
mesiteuō
mes-it-yoo'-o
From G3316; to interpose (as arbiter), that is, (by implication) to ratify (as surety)