Unclear. According to Viljo Nissilä, from an Orthodox Christian male name Sema, from RussianСемён(Semjón). Other Savonian toponyms possibly derived from Orthodox names include Joroinen, possibly from RussianЕрофей(Jerofej).
Alpo Räisänen disputes this and derives the name from the endonym of the Sámi people (see Proto-Samic*sāmē, especially compare Inari Samisäämi) + the derivational suffix -nki.
Sirkka Paikkala, editor (2007) Suomalainen paikannimikirja [Book of Finnish Place names] (in Finnish), Helsinki: Karttakeskus, Kotimaisten kielten tutkimuskeskus, →ISBN