a signal or message that a ship or plane sends when it needs help immediately （船舶或飛機發出的）緊急呼救信號 to send an SOS 發出緊急呼救信號 an SOS message 緊急呼救信號
a serious request for immediate help 緊急求助的請求
Word Originearly 20th cent.: letters chosen as being easily transmitted and recognized in Morse code; by folk etymology an abbreviation of save our souls.