When you are excited about something.
Or
When when you don't believe someone will keep a promise
Beyonce is coming to Ghana next month
Saa!
I'll buy you an iPhone soon
Saaaaa. Hmmm
saa duuh
person A: Hwy what up
person B: saa duuh
person A: what?
person B: saa saa duuh
Person A: ohhhhh