-Bye Maria, see you later.
It is derived from the italian word "Ciao", a salute that means either "Hello" or "Goodbye". However, in Brazil and Portugal, it is rarely used as an initial greeting - instead, it is a common substitute for the word "Adeus" - "Goodbye" in Portuguese.
The word was largely spread by italian emigrants who brought their culture, including linguistic terms, to all over the world, especially to the South American countries - Colombia, Brazil, Uruguai, Bolivia, Argentina, Venezuela, etc. It is still a very common word in Brazil.
In Portugal, the word is still in usage but it's not as common as in Brazil. In fact, in the Portuguese slang, the word "Tchau" has acquired a slighty different meaning from the original, since it is often used to respond with amusement or fright to a fantastic or unexpected statement, very much similar to the english expressions "Get out of here" or "You don't say so...".
- Tchau, Diana. Vemo-nos amanhã.
- Tudo bem, João. Tchau.
- Tchau, Diana. We'll see each other tomorrow.
- Allright, João. Tchau.
Slang meaning (used more often in Portugal, altough the common usage is still as a substitute for "Adeus"):
- Imagina que a tua namorada tinha lá aparecido de repente.
- Tchau! (Bate na madeira três vezes).
- Imagine that your girlfriend would have suddenly shown up there.
- Tchau (Knocks wood three times).