An Etymological Dictionary of the German Language/Münster
Münster, n. and m., ‘minster, cathedral,’ from. MidHG. münster, OHG. munustiri, munistri, n., prop. ‘monastery, convent’ (OHG.), then (MidHG.) ‘monastery or convent church, collegiate church, cathedral’; comp. AS. mynster, E. minster. Formed from Lat. and Gr. monastêrium, ‘monastery,’ whence also ModFr. moutier, ‘monastery, convent’ (comp. also OIr. munter, manister, as well as OSlov. monastyrĭ, ‘monastery’). MidLat. monasteria were orig. cellae in quibus unicus degit monachus, then generally ‘monastery,’ finally ‘cathedral,’ quod plerisque in ecclesiis cathedralibus monachi, non ut hodie canonici, olim sacra munera obirent (this signification is found even towards the end of the 11th cent.). Mönch was borrowed at the same period as Münster; comp. Abt and Probst.