A risky job or operation, as in The university press is not allowed to either make or lose money—that's a high-wire act. This expression alludes to the aerialist performing on a tightrope stretched high above the ground. [Colloquial; mid-1900s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.