The Hobbit - Chapter 2 - An inquiry

[Anaarkhal]

In ROAST MUTTON the party comes across a group of trolls. After Bilbo works up the courage to burgle the unsuspecting troll, we read, "...Bilbo plucked up courage and put his little hand in William's enormous pocket. There was a purse in it...Trolls' purses are the mischief and this was no exception. 'Ere, 'oo are you?' it squeaked, as it left the pocket; and William turned round at once and grabbed Bilbo by the neck..."

What in the hell is meant by "trolls' purses are the mischief and this was no exception"?

I did find this interesting while searching it elsewhere:

When I first read the "Hobbit", the talking purse did not surprise or disturb me, as it seemed familiar for some reason. Not until I saw this post did I remember where I saw this before. It was in "Jack and the Beanstalk", with the Giant's golden harp crying out when Jack stole it, a parallel to the pinching of the troll's purse. It seems that Tolkien was using one of the features of the "fairy tale" genre. And why not; the "Silmarillion" has a talking dog, sword, and dragon.

Edit: Forgive me if there is a thread over this topic. I searched using the search feature and couldn't find anything.