explain away
explain away {v.}
To explain (something) so that it does not seem true or important.
John explained away his unfinished homework by showing the teacher his broken arm in a cast.
It is hard to explain away Abraham Lincoln's dream about being dead, which he had a few days before he was shot.
The man could not explain away the gun and the marked money from the bank robbery that the police found in his car.
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Source: A Dictionary of American Idioms