This paper addresses the issue of the historical development of for fear (that) in English – a prepositional subordinator ushering in finite clauses of purpose in which negation is inherently coded, i.e. the content of the subordinate clause is negated by the complementiser which does not contain a negative particle in itself. The rise of this construction is studied within the theory of grammaticalization and it turns out to be a regular case of grammaticalization following the mechanisms of grammaticalization such as desemanticisation, extension and decategorialisation.