/ ˌes əʊ ˈes; NAmE oʊ/ noun [ sing. ]
1.
a signal or message that a ship or plane sends when it needs urgent help :
to send an SOS
an SOS message
2.
an urgent request for help :
We've received an SOS from the area asking for food parcels.
—see also Mayday
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WORD ORIGIN
early 20th cent.: letters chosen as being easily transmitted and recognized in Morse code; by folk etymology an abbreviation of save our souls .