In this paper, we discuss the properties of negation in the Oceanic language Äiwoo. Like many Melanesian languages, Äiwoo has a bipartite negation construction; more unusually, this construction contrasts with a simple negation construction involving only the first morpheme of the bipartite negation, which typically has the reading 'not yet'. We account for this situation by arguing that the second part of the bipartite negation originates in a morpheme indicating that the verbal action is unachieved; and that the absence of this morpheme, even after the bipartite negation grammaticalized as the standard negation construction, is typically interpreted as meaning that the event, though unachieved at present, may still take place at some later stage. We further discuss scope effects related to the second negative morpheme as a possible explanation for the cases where single negation does not get the reading 'not yet'. Finally, we argue that Äiwoo shows a variation on Jespersen's cycle not previously discussed in the literature, where the original single negation construction is not lost in the process of grammaticalization, but is rather reinterpreted as indicating what we call 'weakened negation' in contrast to the new double-marked construction.