if milk or other food is off, it has gone sour or bad and no longer tastes or smells right
Example
I think this milk's off — it smells a bit funny.
be off — 2 meanings
One phrase, 2 meanings. Learn them one at a time, then test yourself.
phrase2 meanings
Meaning 2
be completely wrong
If your idea, guess or impression is off, it is inaccurate or not quite right.
Example
A: Did you guess the right number?
B: No, I was totally off.
Tell it apart
Turn this into speaking practice
Pick a meaning and say it out loud. Save the ones you mix up.