Meaning

noun an internationally recognized distress signal in radio code

noun the syllable naming the fifth (dominant) note of any musical scale in solmization

originFrom interpretation of the signal (· · · — — — · · ·), chosen merely as a sequence that is easy to recall and transmit, as normal Morse letters. It is not, as is commonly believed, an abbreviation for "save our souls", "save our ship", or any other phrase.

Etymology from Wiktionary, CC BY-SA 3.0