What does Thrasymachus accuse Socrates of when stating he argues like an informer?

Question

In Benjamin Jowett's translation of The Republic, Thrasymachus accuses Socrates of arguing like an informer (340):

You argue like an informer, Socrates.

And later:

Indeed, Thrasymachus, and do I really appear to you to argue like an informer?

Certainly, he replied.

And do you suppose that I ask these questions with any design of injuring you in the argument?

Nay, he replied, "suppose" is not the word—I know it; but you will be found out, and by sheer force of argument you will never prevail.

I shall not make the attempt, my dear man; ...

And do you imagine, I said, that I am such a madman as to try and cheat, Thrasymachus? I might as well shave a lion.

Why, he said, you made the attempt a minute ago, and you failed.

What exactly is being referred to here? Furthermore, in what way does Thrasymachus claim that Socrates cheated earlier?

Answer 1

I think that the word "informer" has here its usual meaning: a spy, a person who pretends to be on your side, but secretly intends to harm you. This interpretation will account well for Socrates's reactions to Thrasymachus's accusations (emphases mine):

And do you suppose that I ask these questions with any design of injuring you in the argument?

And do you imagine, I said, that I am such a madman as to try and cheat, Thrasymachus? I might as well shave a lion.

(I don't see how the alternative translation "pettifogger" can account for these forceful phrases)

The immediate context of the word "informer" seems to be, that Thrasymachus suddenly realizes that Socrates is interpreting him uncharitably. Instead of interpreting him in a way that will make his assertions truthful and apt, Socrates explicitly prefers to attribute to Thrasymachus an inferior view that will make him open to attack. Socrates is thus revealed as not the friendly partner that he pretended to be.

Tell me, Thrasymachus, I said, did you mean by justice what the stronger thought to be his interest, whether really so or not?

Certainly not, he said. Do you suppose that I call him who is mistaken the stronger at the time when he is mistaken?

Yes, I said, my impression was that you did so, when you admitted that the ruler was not infallible but might be sometimes mistaken.

You argue like an informer, Socrates.

Answer 2

As recommended by Mauro ALLEGRANZA, looking at some different available translations sheds light on what might be the intended meaning here.

In James Adam's translation, rather than 'informer', the word 'pettifogger' is used (emphasis mine):

That is because you argue like a pettifogger, Socrates. Why, to take the nearest example, do you call one who is mistaken about the sick a physician in respect of his mistake or one who goes wrong in a calculation a calculator when he goes wrong and in respect of this error? Yet that is what we say literally—we say that the physician erred and the calculator and the schoolmaster. But the truth, I take it, is, that each of these in so far as he is that which we entitle him never errs; so that, speaking precisely, since you are such a stickler for precision, no craftsman errs. For it is when his knowledge abandons him that he who goes wrong goes wrong—when he is not a craftsman. So that no craftsman, wise man, or ruler makes a mistake then when he is a ruler, though everybody would use the expression that the physician made a mistake and the ruler erred. It is in this loose way of speaking, then, that you must take the answer I gave you a little while ago. But the most precise statement is that other, that the ruler in so far forth as ruler does not err, and not erring he enacts what is best for himself, and this the subject must do, so that, even as I meant from the start, I say the just is to do what is for the advantage of the stronger.

Looking at the dialogue above which follows, a suitable interpretation of 'pettifogger' here would be the second definition of Merriam Webster:

one given to quibbling over trifles

Or Google's definition of 'pettifogging':

placing undue emphasis on petty details; petty or trivial.

Later, in Adam's translation, rather than 'to cheat', the word 'pettifogger' is reused:

“Why, do you suppose,” I said, “that I am so mad to try to try to beard a lion and try the pettifogger on Thrasymachus?” “You did try it just now,” he said, “paltry fellow though you be.” “Something too much of this sort of thing,” said I.

It thus seems that Thrasymachus claims that Socrates 'cheated' earlier by detracting from his argument by intentionally focusing on (misinterpreting?) minor details.