An Etymological Dictionary of the German Language/Monat
Monat, m. (with ModHG. ô from MidHG. â before a nasal, as in Mohn, Mond, Brombeere, ohne, &c.), from the equiv. MidHG. mânôt (d), OHG. mânôd, m., ‘month’; comp. Goth. mênôþs, AS. mônað, E. month. The common Teut. term mênôþ-, ‘month’ (pre-Teut. mênôt-), seems identical with Mond, OTeut. mênin-. The computation of time according to the periods of the moon is perhaps OAryan, since the terms for month in the Aryan languages agree approximately. See under Mond.