Sos

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early 20th century: letters chosen as being easily transmitted and recognized in Morse code; by folk etymology an abbreviation of save our souls .

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etymonline

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SOS

1910, from International Morse code letters, chosen arbitrarily as being easy to transmit and difficult to mistake. Not an initialism (acronym) for "save our ship" or anything else. Won out over alternative suggestion C.Q.D., which is said to mean "come quickly, distress," or "CQ," general call for alerting other ships that a message follows, and "D" for danger. SOS is the telegraphic distress signal only; the oral equivalent is mayday.