grasp at straws

[grasp at straws] or [clutch at straws] {v. phr.} To depend onsomething that is useless or unable to help in a time of trouble ordanger; try something with little hope of succeeding.

To depend onyour memory without studying for a test is to grasp at straws.

Therobber clutched at straws to make excuses. He said he wasn't in thecountry when the robbery happened.