Where does “Itajaí-Açu” come from?

Itajaí-Açu (Portuguese) comes from Portuguese -açu, from Old Tupi -gûasu, from Proto-Tupi-Guarani *-wat͡ʃu — augmentative suffix.

Ancestry of “Itajaí-Açu”, step by step

StepLanguageWordMeaning
1Portuguese-açuforms augmentative of nouns
2Old Tupi-gûasucombining form of gûasu; big
3Proto-Tupi-Guarani*-wat͡ʃuaugmentative suffix
Every word from Proto-Tupi-Guarani *-wat͡ʃu →Every word from Old Tupi -gûasu →Every word from Portuguese -açu →