Longman Dictionary of Contemporary English homepage

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"

SOS

 noun
     
Related topics: Broadcasting, Telephone, Telegraph
SOS [singular]
1TTTCT a signal or message that a ship or plane sends when it is in danger and needs help [↪ mayday]
2TCB an urgent message that someone is in trouble and needs help:
This is an SOS for a Mr. Tucker, whose mother is seriously ill.
Definition from the Longman Dictionary of Contemporary English
Advanced Learner's Dictionary.

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