Lewis & Short

a-stĭpŭlor (adst-, Weissenb., Jan), āri, 1, v. dep. (act. adstĭpŭlo, āre, Jul. Val. Rer Gest. Alex. M 1, 18), to join in a stipulation, to stipulate with, Gai Inst. 3, 112.
Trop, to agree with one, = adsentiri: adstipulari irato consuli, Liv. 39, 5: Hellanico adstipulatur Damastes memorans, etc., Plin. 7, 48, 49, § 154.