tulli + saari, based on a mistaken translation of the Swedish name Tulholm (variant of the current Swedish name Turholm) by interpreting its initial component as tull. The Swedish name may go back to an earlier *Tulholm, *Tuloholm, which could originate from an old Swedish name Tule or from dialectal tule(“rascal”), or from an old Finnish name *Tuulossaari (tuulos- + salo) or *Tuulisalo (tuuli + salo).[1]
^ Sirkka Paikkala, editor (2007), Suomalainen paikannimikirja [Book of Finnish Place names] (in Finnish), Helsinki: Karttakeskus, Kotimaisten kielten tutkimuskeskus, →ISBN