Rawdon Crawley, Esquire, gave vent to a prodigious whistle, in token of astonishment at this announcement. He couldn't deny it. His father's evident liking for Miss Sharp had not escaped him. He knew the old gentleman's character well; and a more unscrupulous old--whyou--he did not conclude the sentence [...]
When he saw Rebecca alone, he rallied her about his father's attachment in his graceful way. She flung up her head scornfully, looked him full in the face, and said, "Well, suppose he is fond of me. I know he is, and others too. You don't think I am afraid of him, Captain Crawley? You don't suppose I can't defend my own honour," said the little woman, looking as stately as a queen.
"Oh, ah, why--give you fair warning--look out, you know--that's all," said the mustachio-twiddler.
"You hint at something not honourable, then?" said she, flashing out. (14.42-48)
Rawdon doesn't complete the sentence about his father, but we easily can. "Something not honorable" could mean one of two things: 1) Becky becoming Sir Pitt's mistress, which is unlikely, since she's all about putting a ring on it; or 2) getting assaulted by Sir Pitt, which has some traction based on his earlier threat to keep coming to her room every night if she doesn't put her candle out sooner. Either way, Becky's instant coming to the point rather than pretending she doesn't understand what Rawdon is talking about gets him kind of hot and bothered in his jealousy of his father.