We often say it loudly so people can hear your true smagu bagu culture!
used to show that you not being serious and are joking, usually used to show anger aswell, if you are afraid to say you are angry you say smagu bagu.
its a very funny remark may i add.
also smagu de bagu is used aswell... say it in funny accents and make it fun!.
'Do you know the muffin man?'