Where does “Tàmesi” come from?

Tàmesi (Catalan) comes from Latin Tamesis, from Proto-Brythonic *Tamesis.

Ancestry of “Tàmesi”, step by step

Step	Language	Word	Meaning
1	Latin	Tamesis	the Thames a river in Britain that runs through London
2	Proto-Brythonic	*Tamesis	—