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 modem 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.