Slovene naj (coarsely: ‘let; should’) is a highly multifaceted modal element. In the contemporary standard language, it occurs as a modal marker and figures in the formation of periphrastic predicates and complex clauses. With regard to the latter, naj has also been analysed as a clausal complementiser. In order to get a clearer understanding of its potential to contribute to complex clause formation, the present paper traces the development of naj from the earliest sources of the 16th century onwards. Carving out the semantic and syntactic changes underlying the emergence of its remarkable polyfunctionality, the features of ‘non-assertion’ and ‘speaker-attitude’ turn out as central semantic components. Both relate to the original imperative function of naj, which got lost in the course of its development. This semantic bleaching was accompanied by a functional expansion and an accumulation of structural options for naj. Whether these options include the function as a clausal complementiser emerges as an empirical question that needs to be discussed against the more general background of linguistic categorisation.