I suppose not

[Winwin2011]

John: We can't just leave without telling anyone.
Jenny: I suppose not. (Does it mean that she does not agree with what John has said?)

Thanks for your help!

 

[Barb_D]

It means "You're right. I agree that we should not do that." (That=leave without telling anyone.) Often it comes with a feeling of regret.

We can't eat the entire cake Mom made for dinner tonight.
No, I suppose not. (No, you're right. Eating the entire cake is a bad idea.)

You can't just waltz in there and say you want a raise!
No, I suppose not. (You're right. Doing that is a bad idea.)

 

[BobK]

:up: But there's also, often, the implication that the speaker would prefer not to do what they admit they shouldd:

A: We can't leave without telling anyone.
B: I suppose not - but I really feel lousy we should never have come.

b