Believe it of any of you

[Xinfu]

p.243
-I would never have believe it of any of you. Mr. Filch says you were up the astronomy tower. It's one o'clock in the morning. Explain yourselves.

What's the meaning of 'of'? 'believe...of'?

[Rusty]

I would never have believed it of any of you.

You've most likely surmised that the sentence is stating someone's disbelief that others acted in an unexpected way.

The phrasing is quite common and used correctly (with the correction). The first 'of' could be replaced with 'about' and mean exactly the same thing (believe something about someone).

Another way to say the phrase is 'I would never have believed your behavior possible'.

[Xinfu]

Thank you.