The article's objective is to provide a proper etymology of the Polish word 'zas'. Linguists present two different and contradictory theories; one claims 'zas' comes from the neuter form of the old demonstrative pronoun si (za+sie>zasie>zas), others think it comes from the reflexive pronoun 'sie (za+sie>zasie>zas). The analysis of dictionary material allows to assume there were two distinct words, which lost their separate meanings because of the phonetic similarity and are continued by one lexeme 'zas'.