What was Big Willy Shakes going for?
Shh! We've got a secret to keep. That's where this phrase comes in. Hume isn't talking about his mum or those guys wrapped up in toilet paper. Nope, he's using the Middle English form of the word "mum" which meant to make silent. It comes from the Latin word "mutus" where we get the word "mute" (which is really the same thing).
Now that we know what the word means, let's plug it back in to the passage. Hume tells us "Seal up your lips and give no words but mum." He's really saying that he'll be silent because no words will come out of his trap.
Now Hume knows how to play the field to get what he wants, so he sets up Eleanor's downfall by bringing in witches and then tipping her enemies off to it. We think we're supposed to see a slimy guy in this scene. He is more interested in the gold than anything else.
So whether you know what's to come in Henry VI, Part II, or you're just really good at reading people, we're betting you can see that Hume's little comment about keeping everything hush-hush isn't how it actually goes down. He might promise to keep quiet, but really, he helps anyone who will give him some gold.
It's a funny idea, though, isn't it? Telling us his word will be "mum" (a.k.a. he'll be talking) instead of just saying he'll be silent from now on almost hints that he will have some words to say.
Back in Shakespeare's day, mummer's plays were sometimes performed before the real play got started. These were little skits with players who didn't speak but acted out roles. They, too, were mum. Shakespeare's audience might have picked up on this similarity to what's going down in the play here. Hume is playing a part—while silent. Even though he promises to help Eleanor, he's only interested in helping himself. So, his silence to her about his real plan is almost like a mummer (or silent role).
It turns out others weren't so silent about this phrase either. The same idea was first recorded back in 1376 in Piers Plowman as "then get a mom of her mouth til money were shrewd" and again in The Comedy of Acolastus in 1540: "I dare not to do so much as put my hand to my mouth, and say mum, is counsel."
Still, it seems like Shakespeare liked the idea of his characters saying "mum," so he wrote it for them again in King Lear, The Taming of the Shrew, Richard III, Much Ado About Nothing, The Merry Wives of Windsor, Measure for Measure, and The Tempest. Wait a minute. Isn't the word supposed to be about keeping quiet?