The Hobbit - Chapter 2 - An inquiry
 

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

*shrug* Pretty self-explanatory, I'd have thought: the purse is enchanted– either actually sentient, or equipped with a sort of magical alarm. Is your problem with this the fact that it doesn't fit with what we're elsewhere told about Middle-earth trolls?

Troublesome article
 

I've never seen or heard the phrase "x is the mischief" anywhere else either, and I'm pretty sure that a proof reader or Tolkien himself would have spotted this if it had been a mistake. I'm reasonably sure that he was using 'mischief' as a synonym of 'trouble' or 'problem', but it could also be a joke.

'Mischief' isn't a typical Tolkienian word, since it's of Norman French origin. Its original form in Old French was meschever, meaning roughly 'come to a bad end', and it bears no relationship with English 'chief', which derives from chief/chef. It sounds similar, however, and in a story for children it's a harmless enough joke to suggest that there can be a mis-chief, which is the foremost misfortune. In the circumstances, that's not an unreasonable description of the purse Bilbo tries to steal.

I always assumed it was a substitute for the Devil implying fiendish difficulty rather than impossibility as in someone had the Devil's own job doing something. Now in LOTR the Rohirrim talk a bit a about devils and devilry but I thougth that a figure of speech was not suitable either because a children's book or just because it didnt' fit in with ME.

Only example of anything similar I can find is:

"He is all pine and I am apple-orchard.
My apple trees will never get across
And eat the cones under his pines, I tell him.
He only says, "Good fences make good neighbors."
Spring is the mischief in me, and I wonder
If I could put a notion in his head:"
Robert Frost: Mending Wall http://www.bartleby.com/104/64.html

The plot thickens
 

That was well found, but it's still following the normal usage of 'mischief' as established in the seventeenth century and found in any dictionary. Something tells me that Tolkien was up to something here, and a glance at A Middle English Vocabulary confirms this. In his gloss for myschefe and its variant spellings, Tolkien gives the Old French form as mesch(i)ef, clearly identifying it with chef. It would be just like him if his research into a word's origins affected his use of it in modern English.

There is the expression "do yourself a mischief" meaning injure or harm yourself.
Not all phrases are well documented. My mother used to call the little triangular patches of grass at a fork in a lane "god cakes". I tried to look it up and it took a while - I began to think it was a "motherism" but I then found there was a little triangular local pastry in her native Warwickshire called a god-cake ,,, but it took a bit of digging and even then the only refs so far are in a cookery blog and a Leaminton Parish magazine...

I don't see the issue here.

One of the definitions of the word mischief is "harm or trouble, especially as a result of an agent or cause". I think that is entirely in line with the intention of the sentence, saying "Trolls purses are the trouble". I think Tolkien just used what to us appears to be an unconventional manner of speaking (as he does through much of his works!), though it's entirely clear and correct from a grammatical view.

Oh, right, I didn't get the point of your enquiry.

What the others have said: literally the phrase is just a more colourful way of saying: "...are the harm or trouble." But I'd say Mith is also right that it's really a euphemism for "...are the devil" (which you probably wouldn't be allowed to say in a kid's book of that time).

And it *is* an actual, documented expression.

Well done Nerwen ...there in perhaps the most obvious place all the time... I wonder if it was one of "his" definitions though I have a vague idea Tolkien worked on a different part of the alphabet at the OED...

Strangely enough, this exact line is quoted by Tom Shippey as an example of Tolkien's 'of course...' style of narration in The Hobbit. I should have checked my old 1950s Concise Oxford: these modern dictionaries are poor efforts.