Where does “zusammensein” come from?

zusammensein (German) comes from German Sein, from Middle High German sein, from Old High German sīn, from Proto-West Germanic sinn, from Proto-Germanic sinnaz, from Proto-Indo-European sentnós, from Proto-Indo-European sent-, from Proto-Indo-European h₁es- — to be.

Ancestry of “zusammensein”, step by step

zusammensein traces back through two separate lines of ancestry.

via German Sein

StepLanguageWordMeaning
1GermanSeinexistence, being, essence
2Middle High Germansein
3Old High Germansīnhis; to be
4Proto-West Germanicsinnsense, perception; sense, meaning; reason, mental...
5Proto-Germanicsinnazsense, perception; sense, meaning
6Proto-Indo-Europeansentnós
7Proto-Indo-Europeansent-to head for, to go, to travel; to feel
8Proto-Indo-Europeanh₁es-to be

via German zusammen

StepLanguageWordMeaning
1Germanzusammentogether, jointly
2Middle High Germanzesamenetogether
3Old High Germanzisamanetogether
Every word from Proto-Indo-European h₁es-Every word from Proto-Indo-European sent-Every word from Proto-Indo-European sentnós