Inherited from Proto-Italic *Menerwā, from Proto-Indo-European *menesweh₂, extended from *ménos (“mind”), from *men- (“to think”). Compare Sanskrit मनस्विन् (manasvín, “full of mind or sense”).
Another hypothesis by Clayton (2024) suggests derivation from Proto-Indo-European *ménwr̥ (“thought”), from the same root above.