cassa?

IN the first verse of the metta sutta (Sn 1.) there is this phrase:

suuvaco cassa mudu anatimaanii

I can't find the word "cassa" in any dictionary. I think I can see
from various translations what it might mean, but I'm interested to
get a more authorative answer.

BTW I'm using the chat.t.ha san.gaayana CD as my source.

Any help appreciated.

Regards
Michael