sure as a pickle 

Being 'sure as a pickle' means not being very sure at all. Using this phrase often gives a sense of false hope, as it sounds like an optimistic phrase while actually being quite pessimistic.

Pickles are very unsure organisms, as shown by their common trait of rolling around on tables. This behavior is caused by the existence of their many bumps, which bother them to no extent. They feel bad for each and every one of these bumps, which they must lie on and crush, no matter which way they roll. Thus, they must keep moving.

Nothing is ever less sure than a pickle.

Person 1: So, is our plan still on for tonight?
Person 2: Um, probably.
Person 1: You sure?
Person 2: I'm as sure as a pickle!
The next day...
Person 1: You didn't show up! What happened?
Person 2: I told you, I was as sure as a pickle, which is really not very sure at all.

sure as a pickle by Rolls With Pickles August 4, 2009