a signal or message that a ship or plane sends when it needs help immediately （船舶或飞机发出的）紧急呼救信号 to send an SOS 发出紧急呼救信号 an SOS message 紧急呼救信号
a serious request for immediate help 紧急求助的请求
Word Originearly 20th cent.: letters chosen as being easily transmitted and recognized in Morse code; by folk etymology an abbreviation of save our souls.