Some of the passengers began to pray under their breath, in a feeble attempt to suppress their panic.

feeble 是形容词，意思是"虚弱的、无力的"，常用来形容身体虚弱或努力/尝试效果微弱。词源来自古法语"feble"，与拉丁语"flebilis"（可悲的）相关。在句中形容这种祈祷的尝试效果微弱，难以真正压制恐慌。