mynä- + -mä. mäki(“hill”) has later been introduced folk-etymologically through the inflected forms. The origin of the root is unknown, but related to Mynäjärvi, the name of a nearby lake (whence the name of the nearby river Mynäjoki). First attested in 1567 as Mynemedh;[1] this indicates that the name was originally in the plural (*Mynämät). The genitive form *Mynämäẟen > Mynämäen then served as the reason to introduce mäki (genitive mäen). The root is found nowhere else in Finland, which is indicative of a loan; thus, according to Heikkilä, it could have been borrowed in the 12th century from Early Middle High Germanmünech(“monk”) (> *Mynelahti(literally “monk('s) bay”) > *Mynämät).[2]
The internal locative cases (inessive, illative and elative) are used with this place name when referring to a location; for example, "in Mynämäki" is Mynämäessä. External locative cases (Mynämäellä) are also accepted.
^ Sirkka Paikkala, editor (2007), Suomalainen paikannimikirja [Book of Finnish Place names] (in Finnish), Helsinki: Karttakeskus, Kotimaisten kielten tutkimuskeskus, →ISBN
^ Heikkilä, Mikko (2016) Kuka oli herra Heinäricki? – piispa Henrikin arvoitus (in Finnish), Tampere University Press, →ISBN, pages 77–85