samen.

From Middle Dutch tsamen, a contraction of earlier tesamen, from te samen. Although not clear from the spelling, the ⟨s⟩ in the Middle Dutch word was pronounced as voiced /z/. That is, (te)samen was pronounced /(tə)zaːmən/, like the modern descendant tezamen and the related words gezamenlijk and verzamelen. The contracted form tsamen, however, had a voiceless /s/ because it was assimilated to the preceding /t/, creating an affricate. This affricate /ts/ was later simplified to /s/. Middle Dutch samen derives from Old Dutch saman, from Proto-West Germanic *saman, from Proto-Germanic *samanai (“together”), from Proto-Indo-European *sem- (“one, together”). Compare also sommige and soms, from the zero grade of the same Indo-European root.