It has long been noticed that the Slavic superlative prefix nai‐/naj‐ comprises two components: *na + *i. The former can be identified with the preposition Sl na ‘on(to)’ which developed an intensifying meaning when used as a prefix. The origin of the second component, on the other hand, has not been determined satisfactorily so far. This paper argues that it can be identified with the Slavic definiteness marker. Drawing upon a typologically common pattern, it is supposed that at a pre‐stage of Slavic superlative was derived from the comparative with the aid of a definiteness marker. Definiteness of a noun phrase in earlier Slavic and its neighbouring branch Baltic was denoted by adding a definiteness marker of pronominal origin to the right edge of the adjective. As data from Old Lithuanian show, this morpheme was originally not bound to the position following the adjective. When an adjective/participle was preceded by a prefix/preverb, it could be placed between these two elements. I argue that this state of affairs is also reflected in the Slavic superlative prefix naj‐/nai‐. Based on the proposed scenario, the article discusses certain general issues related to the externalization of trapped inflectional morphology and the phenomenon of endoclisis.