*aistô

From a pre-Germanic base *h₂oyd-sth₂-o-, ultimately from Proto-Indo-European *h₂eyd- (also reflected in *aitaz and *aitrą), with o-grade of the root and an -sth₂-o- enlargement (Kroonen 2013: 14). The word was probably secondary in Proto-Germanic: neuter n-stems denoting body parts remained productive in Old Norse, so Old Norse eista need not continue an inherited PIE n-stem (Kroonen 2011a: 44–45; Kroonen 2013: 14). Cognate formations occur in Balto-Slavic: Lithuanian ìnkstas and Old Church Slavonic исто (isto), for which Derksen reconstructs *h₂id-st(h₂)-es-/*h₂id-st-es-, indicating a closely related but not morphologically identical formation (Derksen 2008: 215; Derksen 2015: 202) . The same root is reflected in Ancient Greek οἰδέω (oidéō) and οἶδος (oîdos), Old Armenian այտնում (aytnum), and Latin aemidus (Beekes 2010: 1053–54; Kroonen 2013: 14; Martirosyan 2010: 61, 706; Orel 2003: 10; Rix et al. 2001: 258).