etymology - Meaning of "couple o' pins" in this poem by Tolkien

Here's part of a poem from Tolkien:

'For a couple o' pins,' says Troll, and grins,
'I'll eat thee too, and gnaw thy shins.
A bit o' fresh meat will go down sweet!
I'll try my teeth on thee now.

When you check up the meaning of "pin" in a dictionary, here is the usual definition (all the following definitions are from Google):

a thin piece of metal with a sharp........

The usage in the poem definitely has nothing to do with the previous definition. I suspect it has to be either this:

a person's legs.

Or this:

a half-firkin cask for beer. (British)

It could easily be neither of those as well. So folks, do you have anything to offer ? What does "a couple o' pins" mean in this context ?

Answer

I think this may be a variation on the BrE expression "for two pins", which means:

At the slightest provocation; for the smallest reason.
_{Farlex Dictionary of Idioms (via TFD)}

I assume that "pins" here refers to the type used in sewing or another practically worthless type of pin.

The earliest attestation of this idiom that I found so far is from 1890:

For two pins I'd put a match in every gunyah on the place.
_{The Squatter's Dream: A Story of Australian Life}

Here's another early example from The Times (London, 1794):

I'll blow you up for a sodomite, for two pins.

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There are other, older idioms where pins are worthless (note that the expression most likely refers to pins made of wood or bone, not metal). "Not worth a pin", "wouldn't care a pin", etc.:

He seide al þat he had ywonne
Jn þe werlde vnder sonne,
He nolde ȝiue þere-of a pynne,
Bot he miȝth þise wynne.

He said all that he had won
In the world under the sun,
He wouldn't give thereof a pin,
If he might win this.
_{Kyng Alisaunder}

_{See 9. b for more quotes (and a definition) from Middle English.}

It was even used by Shakespeare:

A round hose, madam, now's not worth a pin.
_{The Two Gentlemen of Verona}

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It's worth mentioning that I found the exact same expression "for a couple o' pins" in:

• The Leisure Hour (1904):
  
  
  
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    [F]or a couple o' pins I'd shteam-rowl yez under the two feet o' me.

    
    

    
    
  
  
  
  


• The chimney corner (1879):
  
  
  
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    See now, for a couple o' pins I'd take both yerself an' the little sweep ye call 'John' to the lock-ups!