。
If it were me, I couldn't say something that brazen.
Literal
I [if-だったら] such brazen thing [topic-は] cannot-say.
ずうずうしい ('brazen, shameless, impudent') is a vivid i-adjective for rude self-assertion. そんなずうずうしいこと = 'such a brazen thing (to say).' The potential negative 言えない ('cannot say') expresses the speaker's inability to do what someone else has done. The contrastive は after こと sharpens the focus on 'a thing like that.'