Reconstruction:Proto-Slavic/jistъ

Cognates include Latvian ĩsts (“true”), also ĩstens, ĩstans, as well as Latin Istaevōnēs (name of an ancient Germanic tribe); see the Wikipedia entry on Istvaeones. Further cognates doubtful:

• Vasmer proposes a connection to Latvian īksts and Lithuanian áiškus (“clear”) (also found in the variants ýškus, éiškus, ýskus, ìščias), Old Armenian իսկ (isk, “in reality”), which are in turn related to Russian я́сный (jásnyj, “clear”).
• Stang (quoted in Vasmer) proposes a connection to Sanskrit ईशे (ī́śe, “I am in possession”), ईशान m (īśāná, “lord, wealthy”), Gothic 𐌰𐌹𐌲𐌹𐌽 n (aigin, “property”), 𐌰𐌹𐌲𐌰𐌽 (aigan, “to own, to have”), Old High German eigan (“own”) (whence German eigen, English own).
• Trubachev (quoted in Vasmer) subscribes to Toporov's theory that the original meaning of the Proto-Slavic adjective was "the same" and is cognate with Latin iste (“that (of yours)”), built from Proto-Indo-European *ís (“the”) + *só (“this, that”) (stem *to-; whence English the, that).

*jistъ [1]

1. true, genuine
2. same

• *věrьnъ, *pravъ, *ščirъ, *rěsnъ, *ěsnъ

• *nejistъ, *nejistovъ
• *jistina
• *jistьcь

• Vasmer (Fasmer), Max (Maks) (1964–1973), “и́стый”, in Etimologičeskij slovarʹ russkovo jazyka [Etymological Dictionary of the Russian Language] (in Russian), translated from German and supplemented by Trubačóv Oleg, Moscow: Progress
• Trubačóv, Oleg, editor (1981), “*jьstъjь”, in Etimologičeskij slovarʹ slavjanskix jazykov [Etymological dictionary of Slavic languages] (in Russian), volume 08, Moscow: Nauka, page 246