In episode 1 (Dish and Dishonesty) of Season 3, Edmund Blackadder, addressing Prince George, uses this phrase after an encounter with the Prime Minister. It does not appear to be an idiom, so I assume this had something to do with the conversation they just had:
At Prince's House
Edmund: Your Highness; Pitt the Younger.
George: Why, hello there, young sabre, m'lad! I say, here's one: I've a shiny sixpence here and for the clever fellow who can tell me which hand it's in.
(Pitt just stares.)
G: Hmm? Oh, school, school! On half hols, is it? Yeah, I bet you can't wait to get back and get that bat in your hand and give those balls a good walloping, eh?
E: Mr. Pitt is the Prime Minister, sir.
G: Oh, go on! Is he? What, young Snotty here?
P: I'd rather have a runny nose than a runny brain.
E: Umm, excuse me, Prime Minister, but we do have some lovely jelly in the pantry, I don't know if you'd be interested at all...?
P: Don't patronise me, you lower middle class yobbo! (aside) What flavour is it?
G: I say, Blackadder, are you sure this is the PM? Seems like a bit of an oily tick to me. When I was at school, we used to line up four or five of his sort, make them bend over, and use them as a toastrack.
P: You don't surprise me, sir -- I know your sort. Once, it was I who stood in the big, cold schoolroom, a hot crumpet burning my cheeks with shame. Since that day, I have been busy, every hour God sends, working to become Prime Minister and fight sloth and privilege wherever I found it.
E: I trust you weren't too busy to remove the crumpet...
P: You will regret this, gentlemen. You think you can thwart my plans to bank- rupt the Prince by fixing the Dunny-on-the-Wold by-election, but you will be thrashed! I intend to put up my own brother as a candidate against you.
E: Oh, and which Pitt would this be: Pitt the Toddler? Pitt the Embryo? Pitt the Glint in the Milkman's Eye?
P: Sirs, as I said to Chancellor (Messenec?) at the Congress of Strasbourg: `Pooh to you with knobs on!' We shall meet, sirs, on the hustings. (exits)
G: I say, Blackadder, what a ghastly (squit?)! He's not going to win, is he?
E: No, sir, because, firstly, we shall fight this campaign on issues, not personalities. Secondly, we shall be the only fresh thing on the menu. And thirdly, of course, we'll cheat.
Transcription from allblackadderscripts.blogspot.com.
I'm sure he's referring to something from this conversation, but I can't for the life of me figure out what it is.