While the simple thematic ending in 2sg. is relatively well attested 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 it is really a direct descendant of PIE *-e-si. This paper will revisit a proposal originally submitted by Brugmann (1889) and later by Rasmussen (1999), indicating that the ending has its origin in a reanalysis of the root and ending of the athematic present forms in 2sg. of the roots ending in *-s, e.g., *es-si  > *esi → *es-i ‘you are.’ 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 of approach. The instances where the proposed reanalysis would have happened are limited to a few verbs. Moreover, the new ending that originated in the athematic *es-i more seriously affected the thematic verbs, not the other athematic verbs. The new ending will attempt to modify this original proposal by assuming that the reanalysis could have happened as well in the athematic 2sg. s-future forms, which has enjoyed great productivity since Proto-Baltic prehistoric times.