This is one of four related Middle Korean interrogatives all beginning with /mɨsɨ-/: 므스 (musu), 므슥 (musuk), 므슴 (musum), and 므슷 (musus). The precise etymological relationship between the four remains unclear.

• IPA^(key): /mɨsɨs/

므슷〮 (mùsús)

1. (interrogative) what

   Synonym: 어느 (enu)

   • 1481, 分類杜工部詩諺解 / 분류두공부시언해, page 15:6b:

     므슷〮 이〯리 이시리〮오

     mùsús ǐl-ì ìsìlíwò

     What thing would there be?

• 므스 (musu) is used only for the semantically weak nouns 것 (kes, “thing”) and 일 (il, “work, event”).
• 므슴 (musum) is used generally, but typically in rhetorical questions.
• 므슷 (musus) is used primarily for 일 (il, “event”) but is rarely attested elsewhere.
• 므슷 fails to survive into Early Modern Korean, while 므스 vanishes near the end of the eighteenth century. The default Early Modern interrogative determiner is 므슴 (meuseum), which loses its rhetorical sense and becomes a general interrogative determiner.
• 므슨 (meuseun), probably a hypercorrection of 므슴 (meuseum) motivated by the adnominal ending 은 (eun), is first attested in 1637 and becomes more widespread than 므슴 around 1850, producing Modern Standard Korean 무슨 (museun).

• 이광호 (Yi Gwang-ho) (2009) “'므스'와 '므슥/므슴/므슷'의 의미특성 및 형태변화 [meuseu wa meuseuk/meuseum/meuseut ui uimiteukseong mit hyeongtaebyeonhwa]”, in Gugeo gungmunhak, volume 151, pages 35—57
• 장요한 (Jang Yo-han) (2013) “중세국어 의문사 '므스'류의 교체 양상과 단일화 [jungsegugeo uimunsa meuseu ryuui gyoche yangsanggwa danilhwa]”, in Eoneowa jeongbo sahoe, volume 20, pages 235—259
• 장요한 (Jang Yo-han) (2017) “중세 국어 의문사 '므슴'의 품사 발달과 그 원리 [jungse gugeo uimunsa meuseum ui pumsa baldalgwa geu wolli]”, in Urimalgeul, volume 75, pages 29—50