attorn

Black’s Law Dictionary 1st Edition, page 103:

ATTORN. In fuedal law. To transfer or turn over to another. Where a lord aliened his seigniory, he might, with the consent of the tenant, and in some cases, without, attorn or transfer the homage and service of the latter to the alienee or new lord. Bract. fols. 81b, 82.

In modern law. To consent to the transfer of a rent or reversion. A tenant is said to attorn when he agrees to become the tenant of the person to whom the reversion has been granted.