Or better still, know of a website where it is possible to find the origin of expressions or idioms like that?
I've had a look myself but can't find any other than definitions of what it means, which I already know.
For those who don't know what the expression means, slating someone/something means to criticise someone/something.
e.g. His most recent film was slated by the critics.
For future reference, I have found this etymology dictionary website helpful, except that it didn't include your understanding of the word 'slate':
(see page 49 for the word slate)
We don't use this term in Canada.
... back now after perusing my trusty old dictionary. This is what I found:
Slate: v.t. : to abuse, to review unsparingly
From the Old Norse word via Old English <em>slaeten:</em> to bait.
And just in case there are any native speakers who are confused, I believe this meaning of 'slate' is only British. In US English, if you are 'slated', your name is metaphorically written 'on the slate', meaning that you are in line to assume a particular role or position.
I always find the Urban Dictionary quite amusing when I want to look up the various meanings of words. Who wudda thunked slate could be used so richly?
http://www.urbandictionary.com/define.php?term=slate
A male slag; s'later dudes; that sounded slate; She slates him.
