grasp at straws
grasp at straws or clutch at straws {v. phr.}
To depend on something that is useless or unable to help in a time of trouble or danger; try something with little hope of succeeding.
To depend on your memory without studying for a test is to grasp at straws.
The robber clutched at straws to make excuses. He said he wasn't in the country when the robbery happened.
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Source: A Dictionary of American Idioms
