CQD

Just as 'SOS' is often thought to stand for 'Save Our Souls', 'CQD' is often thought to have stood for 'Come Quick, Danger' or 'Come Quick, Distress'. In fact, neither of them really stood for anything. According to Wikipedia, 'CQD' was created by the Marconi company, based on 'CQ', the signal used on land telegraphs to indicate a message of interest to all stations – which was an abbreviation of the French word sécurité. The 'D' was added to signify Distress. 'CQD' was never an international standard, but was used worldwide by Marconi operators.

What we know as 'SOS' was a German standard, so if it did stand for anything it would be something in German. The main reason for its adoption as the international standard is probably that its Morse code pattern is easier to remember, and more recognisable, than that for 'CQD'. 'SOS' was adopted in 1906 at the second International Radiotelegraphic Convention, held in Berlin.

RMS Titanic is usually cited as the first ship to have used the SOS signal. Wikipedia tells how Jack Phillips, the radio operator on board the Titanic, initially sent 'CQD' – which was still commonly used by British ships. Harold Bride, his junior, suggested using 'SOS', adding half in jest that it might be his last chance to use the new code. Phillips thereafter began to alternate between the two. Though Bride would survive the sinking, Phillips would not.