Algerian Arabic has developed a new indefinite determiner kaš, with a number of loosely related functions. Despite its ubiquity in modern Algiers, this word is absent from 19th century sources, and appears to be a relatively recent development. More recent sources allude to it briefly without ever giving a full description. This article describes its syntactic distribution and its meaning in the dialect of Dellys (north-central Algeria) for the first time, identifying five distinct constructions in which kaš appears. It then examines this form's history based on written data. Based on the results, it reconstructs the reinterpretations that produced the present-day distribution of kaš, showing that it derives from a combination of an existential predicator with the polyfunctional morpheme ši. This finding confirms the existence of a grammaticalisation pathway from existential to indefinite quantifier, a question bearing on some scenarios proposed in the context of the debate within Arabic dialectology over the history of ši/šay'.