His accusations are entirely fictitious with no evidence whatsoever.
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This idiom comes from Han Dynasty writer Sima Xiangru's (司马相如) prose poem 子虚赋 (Zǐxū Fù). The characters 子虚 (Master Fictitious) and 乌有 (Mr. Nonexistent) are names of invented characters in the poem, hence the idiom meaning 'purely imaginary.'