Random Jotting # 024

In Tamizh, people quite often say "oru aathara avasarathukku....".
e.g.: If you are questioned about keeping an additional bottle of water in your car, the response is "oru aathara avasarathukku". Or when you lament the lack of a pharmacy along your daily route, you wonder "oru aathara avasarathukku kooda inga oru marundhu kadai kedayaadha" etc etc.

I can understand the "avasaram" part of the phrase. But where does the "aathiram" fit into the picture?