In Oliver Twist, Dodger refers to Oliver as flash companion. Can't find a definition of this anywhere. What does it mean?

Throughout Oliver Twist, Charles Dickens uses the word flash to describe certain characters in a way that seems unusual today. Flash in this case refers to the world of thieves, tramps, and other social outcasts. This sense of the word is largely obsolete today and is used more often in Great Britain than in other English-speaking countries.

So when the Artful Dodger says to Oliver, "I suppose you don't know what a beak is, my flash com-pan-i-on," he is acknowledging their shared poverty and social rejection. Although flash is used in a brotherly way in this instance, it takes on a more criminal bent elsewhere in the novel.

Throughout the novel, Toby Crackit is often referred to as "Flash Toby Crackit," signifying not his poverty but his skills in thievery, as in this quotation:

"But do you mean to say, my dear," remonstrated the Jew, "that the women can't be got over?"
"Not a bit of it," replied Sikes.
"Not by flash Toby Crackit?" said the Jew incredulously. "Think what women are, Bill."
"No; not even by flash Toby Crackit," replied Sikes. "He says he's worn sham whiskers, and a canary waistcoat, the whole blessed time he's been loitering down there, and it's all of no use."