This paper aims to investigate the discourse functions of Korean interrogative
`mwusun` and its historical development. Based upon about 6 million-word spoken and
written corpus, this paper argues that `mwusun` functions as a discourse marker in
modern Korean and that its two main functions as a discourse marker are to mark
`approximation` and `underestimation`. It is assumed that the basic meaning of
discourse marker `mwusun` is to mark `approximation`, and that all the other meanings
are derived from this basic meaning. Based on its meaning as an interrogative and an
indefinite, `mwusun` indicates the speaker`s uncertainty about a given referent or
situation, used as a discourse marker to avoid (un)intentionally the accurate mention
of a referent or to earn the processing time. Furthermore, the speaker`s
(un)intentional uncertainty about a given referent leads to underestimation about his
or her own opinions. While it softens the speaker`s utterance, it marks the speaker`s
negative attitude or belief towards what is assumed to be true in general or by the
hearer. This paper also attempts to explain the development of `mwusun` as a
discourse marker. It seems that the historically equivocal status of `mwusun` either
as a pronoun or prenominal modifier made it easier to free itself from sentential
structure. In addition, its indefiniteness and the negative implication in its
rhetorical questions also contributed to the development of interrogative `mwusun`
into a discourse marker.