The aim of this article is to examine the development and status of LEST – the most common subordinator introducing negative purpose clauses in Middle English. After presenting the relevant nomenclature of the subject and the etymology of the original structure, I analyse different meanings of LEST, i.e. avertive, in-case, apprehensive and apprehensional epistemic functions as well as its structural development throughout the Middle English period. The data for this study are drawn primarily from the Penn-Helsinki Parsed Corpus of Middle English because of the chronological order of the texts included which should enable tracing potential developments of the studied expression.