grasp at straws
Also, clutch at straws. Make a desperate attempt at saving oneself. For example, He had lost the argument, but he kept grasping at straws, naming numerous previous cases that had little to do with this one . This metaphoric expression alludes to a drowning person trying to save himself by grabbing at flimsy reeds. First recorded in 1534, the term was used figuratively by the late 1600s.