mutunium.

Probably from mūtō (“penis”) + -ium; compare pecūlium (used with a sexual sense in Plautus and Petronius). The derived adjective mutūniātus scans with a short first syllable; Weiss (1996) proposes that this is a case of a geminate being shortened after an unaccented vowel, as seen in sacellus for saccellus (compare mamilla from mamma). The reason for the variation in the vowel in the second syllable is unclear. Weiss proposes the following origin for forms with ū in this family of words: initially, the root was combined with the suffix -īnus, forming an adjective mū̆tīnus. After becoming used as the name of a god, Mutunus Tutunus, the form Mū̆tīnus was altered to Mū̆tūnus under the influence of other deity names ending in -ūnus, such as Neptūnus and Portūnus, and then the ū in the second syllable of Mū̆tūnus served as the basis of ū in the second syllable of forms like mū̆tūnium and mutūniātus.