SOS /ˌes əʊ ˈes $ -oʊ-/ BrE AmE noun [singular]
[ Date: 1900-2000 ; Origin: S and O , letters chosen because they were easy to send by Morse code, but often understood as short for "save our souls" ]
1 . a signal or message that a ship or plane sends when it is in danger and needs help ⇨ mayday
2 . an urgent message that someone is in trouble and needs help:
This is an SOS for a Mr. Tucker, whose mother is seriously ill.