Be in a precarious condition or in a state of suspense. For example, The doctor said her life was hanging in the balance. This expression alludes to the suspended balance scale where an object is placed in one pan and weights are added one by one to the other pan until the two are balanced. [First half of 1400s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.