French phrase
Je ne suis pas sûr.
Meaning
I'm not sure.
When to use it
It expresses uncertainty about a judgment. It is a restrained way to avoid a definite answer and invite confirmation.
Example dialogue
Tu penses que ce vin est bon ?
Do you think this wine is good?
Je ne suis pas sûr. Goûte-le d'abord.
I'm not sure. Try it first.