The Old Japanese modal suffix beshi deserves special attention for at least two reasons. Firstly, it is one of only a few modal markers throughout Japanese language history that have both deontic and epistemic meaning, which is very common in English and other Indo-European languages. Secondly, it is said to be extremely polysemous. There might be no other modal suffix in the history of the Japanese language that has been associated with such a range and variety of meanings. There are also concrete examples of beshi that are given divergent interpretations in grammatical analysis and Modern Japanese translations. The primary goal of this paper is to provide a principled explanation both for the different "meanings" of beshi and for divergent interpretations that are due to indeterminacy. It is argued that Old Japanese beshi from a synchronie point of view basically has only a deontic and an epistemic sense, and other "meanings" can be explained either in terms of vagueness or in terms of implicature in specific contexts. Conditions are explained under which indeterminacy between the deontic and the epistemic sense arises and compared with the conditions for indeterminacy and deontic-epistemic polysemy to those observed in the history of modals in English and German. Furthermore an account of the diachronic layering behind the synchronie meaning range of beshi in Old Japanese is given. It is claimed that the development of the meanings of beshi does not strictly adhere to the "deontic-toepistemic" pattern frequently found on Indo-European languages.