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.