Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
istoc, adv. [istic], thither, that way, yonder: quemquam istoc ad vos intromittere, Plaut. Truc. 4, 2, 6: accede illuc: nimium istoc abisti, Ter. Ad. 2, 1, 15; v. also istic.