My eldest known forefather was Heini Kägi born in 1540 and died in 1591. Due to information in the "Staatsarchiv Des Kanton Zürich" (the official Swiss archive) is the first know Kägi Cunradus Kägi born in Villingen Switserland in 1225.
The name originates probably from a celtic tribe called the Kago or Chago (Prof Dr. Hubschmid, Zürich)
Soon you can find more information on www.kagei.nl. I'm working on it.