bye bye, so long, ta ta, see you, etc. [expression]
"Doei" is an informal word that you can use when parting.
Examples of use:
1. Leaving a shop where the interaction was informal, or the staff is informal.
2. Saying goodbye to colleagues at the end of the day.
3. Hanging up the phone after talking to a friend.
Popular use: "doei, doei!".
In the last couple of years it has become popular to say "doei" twice,
especially for women, but men tend to do it too nowadays.