What did Magwitch say that he characterizes as “low?”

The incident referred to here takes place in chapter 40 of the book, when Pip is meeting secretly with Magwitch, whom he has described as his "uncle" to the watchman. Magwitch explains that his real name is Magwitch, and that he was brought up to be "a warmint, dear boy." Pip deduces from what Magwitch says that he has been a criminal and was tried in London.
After he has eaten, Magwitch begins effusively describing how Pip, being "his gentleman," should have "horses to ride, and horses to drive," and appropriate footwear and clothes. He then produces "a great thick pocket-book" and throws it onto the table. He declares that he has returned to England in order to "see my gentleman spend his money like a gentleman." His fervor rises to such a pitch that he stands up and declares, "blast you all!," encompassing the judge who sentenced him as well as the colonist "stirring up the dust." He then says that Pip will be "a better gentleman" than all of these put together.
Pip is embarrassed and upset, in "a frenzy of fear and dislike." At this point, Magwitch apologizes for having said something "low." He then describes himself as having been "low" for many years, and Pip acknowledges this but says he doesn't want to think too much about it. This suggests that the "lowness" he is referring to is his own criminality. The comment he makes about the judge and the colonists was low because it reflects the behavior and attitudes of a criminal, who dislikes the ruling classes and disagrees with the general setup of society, particularly as regards the law. It also may refer to the fact that his money has come as a result of criminal activities. Pip does not seem upset at the fact that Magwitch is discussing money—as, obviously, in these circumstances, they must—but rather at the fact that he leaps to his feet and addresses the room with surly and inappropriate comments about judges and others, suggesting a resentment at the way his "warmint" lifestyle has been treated. In promising to be "low" no longer, Magwitch is saying that he will no longer behave in that way and will try to ensure that Pip can be a real gentleman with the help of the money.

Magwitch is telling Pip of the enormous wealth that he can come to expect from him in addition to the substantial sum he's already received. Magwitch has made it big in Australia, becoming a very wealthy and successful sheep farmer and stock breeder. He wants to continue to shower his good fortune upon Pip, funding his lifestyle as a young gentleman about town. He's so excited at revealing himself to be Pip's benefactor that he takes out his pocketbook and some papers and throws them down on the table. These items contain the details of the substantial wealth that Magwitch wishes to pass on to Pip.
But Magwitch suddenly remembers where he is. He's in the company of a young gentleman and such enthusiasm over one's wealth is considered somewhat vulgar, or "low" in polite society. Magwitch fulsomely apologizes to Pip; he was clearly forgetting his manners. But as someone who's spent most of his life as a convict, Magwitch can be forgiven for his brief relapse.