A particle often expressing a sense of a bit of pity.
It is best not translated into English.
It occurs most frequently in songs and poems and most often seems to be just used as a filler without specific meaning.
The people of old did not regret it, so'osa.
Ga' pe se'osa bakas ta'urungku.
I have not yet given anything.
Asal maka'ase' hep to'o se'osa a'ahin.
Really, the people are truly to be pitied.