Keeping something to yourself B1
When you know more than you're letting on and have to bite your tongue.
hold back B2 — withhold informationA: Did you tell your boss everything?
B: No, I held back some of the problems until I had a solution.
give away B2 — reveal a secretA: Did you tell anyone about my plans?
B: No, I promise I didn't give anything away.
let alone C1 — much less (emphasising something is even less likely or possible)A: Did you finish the whole report last night?
B: I didn’t even start it, let alone finish it!
Don't say a word
A: Go on, you know something about the party. I can tell.
B: I'm not giving anything away, it's meant to be a surprise.
A: Come on, just a hint?
B: Nope. I'm not even telling you the date, let alone who's coming.
Spit it out
A: You're holding something back. What is it?
B: Honestly, I can't say anything yet, it's not been confirmed.
A: Is it about the redundancies?
B: I'm not even meant to know about that, let alone talk about it. So drop it, yeah?