What does John 8:46 mean?

In responding to His critics, Jesus has already pointed out the evidence to which they have access. This includes Scripture (John 5:39–40), miracles (John 8:18; John 5:36), eyewitnesses (John 5:32–33), and His own God-given words (John 8:26). The fact that they reject these is not because they cannot understand, but because they will not understand. They are the spiritual children of the devil, who is a murder and liar (John 8:44). So, they are bound to reject what Jesus teaches. It's the truth, which they "cannot bear" to hear (John 8:43).

At this point, Jesus also brings out another major piece of evidence which supports His claims: His sinless life. If the religious leaders had a single incident which they could use to accuse Jesus of immorality, they would have already brought it out. This is why they resorted to cheap tricks and challenges—they had no other "dirt" to throw. Here, Jesus brings this up directly: none of these men can accuse Him of any sin!

This ties into the hypocrisy of their rejection. If Jesus is attested to by miracles (John 20:30–31), displays a sinless life (Hebrews 4:15), and speaks the truth, why would they not believe Him? The answer will be repeated in the next verse: they are not of God and do not want to hear.