Against the usual assumption that Arabic grammatical operators based on reflexes of šay derive from the Arabic word for 'thing' šayʔ, it is argued here that indefinite quantifiers and partitives instead derive from an existential particle šay that is present in some spoken Arabic dialects of the Arabian Gulf, Oman, and the Yemen. The ambiguity of the existential particle in constructions in which it sets off items in a series lends itself to its reanalysis as a quantifier, and its ambiguity as a quantifier motivates its reanalysis as a partitive. This is consistent with grammaticalization theory, whereby lexical forms give rise to grammatical forms, which themselves give rise to even more grammatical forms. Yet, existential šay likely did not arise from a lexical form. Instead, it is either a borrowing from Modern South Arabian or it is an inherited Semitic feature, ultimately deriving from an attention-focusing demonstrative. Either way, the grammaticalization of a quantitative šī/šē/šay cannot have proceeded directly from word 'thing'. To the contrary, the word šayʔ meaning 'thing' can easily derive from an indefinite quantifier or partitive šay, in a process of degrammaticalization.