not to know what to make of
not to know what to make of {v. phr.}
To be unable to decipher; be unable to identify; not know how to decide what something really is.
I got a mysterious letter asking me to meet Santa Claus at 6 P.M. at the supermarket. Is this a joke? I don't know what to make of it.
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Source: A Dictionary of American Idioms