While the simple thematic ending in 2sg. is relatively wellattested in the Indo-European languages, pointing to the Proto-Indo-European (PIE) *-e-si, it is attested with an unusual form *-ēi in Baltic. Several previous studies have argued how Proto-Baltic (PB) *-ēi could be derived from the PIE *-e-si or whether itis really a direct descendant of PIE *-e-si. This paper will revisit aproposal originally submitted by Brugmann (1889) and later byRasmussen (1999), indicating that the ending has its origin in areanalysis of the root and ending of the athematic present formsin 2sg. of the roots ending in *-s, e.g., *es-si  > *esi → *es-i ‘youare.’ The new ending replaces the inherited ending *-e-si with *-e-i, resulting in the new simple thematic ending *-ei in Baltic.However, some problems have been pointed out with this line ofapproach. The instances where the proposed reanalysis wouldhave happened are limited to a few verbs. Moreover, the new ending that originated in the athematic *es-i more seriouslyaffected the thematic verbs, not the other athematic verbs. Thenew ending will attempt to modify this original proposal byassuming that the reanalysis could have happened as well in theathematic 2sg. s-future forms, which has enjoyed great productivity since Proto-Baltic prehistoric times.