Fisq refers here to serious religious or moral misconduct short of unbelief. It affects a narrator's probity, since reliability in transmission depends not only on memory but also on trustworthiness in conduct.
An unsupported accusation or sectarian impression is not enough. The nature of the criticism, its proof, the narrator's actual behavior, and its bearing on transmission must be evaluated.
Lying versus fisq: deliberate fabrication in prophetic transmission is a specific, exceptionally grave breach; fisq is a broader category of misconduct and cannot be treated as synonymous with fabrication.