Abstracted as a singular, with independent meaning, from the plural iekšas (“guts, entrails”), from an earlier (and still dialectal) iešas with an epenthetic k, from Proto-Baltic *en-styā-s, derived from Proto-Indo-European *h₁en (“in(side)”). The adverb iekšā (“inside”) is the corresponding locative case form; in 16th- and 17th-century sources an old illative form iekšan is also used (from which is derived the old-fashioned preposition iekš). Cognates include Lithuanian įšcios (“(pl.) mother's lap; depth”), Old Prussian instran (“lard”), Old Church Slavonic ѩтро (jętro, “liver”), Old Norse istr (“inner fat”), Middle Low German inster (“slaughtered animal entrails”), Ancient Greek ἔντερον (énteron, “guts, intestines”).