off balance
off balance {adj. phr.}
1. Not in balance; not able to stand up straight and not fall; not able to keep from turning over or falling; unsteady.
Never stand up in a canoe; it will get off balance and turn over.
Paul was speeding along on his bicycle, when an unexpected hole in the road caught him off balance and he fell over.
2. Not prepared; not ready; unable to meet something unexpected.
Our quarterback kept the other team off balance by changing often from line plays to passes and tricky end runs.
The teacher's surprise test caught the class off balance, and nearly everyone got a poor mark.
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Source: A Dictionary of American Idioms