Under great stress, as in I was on the rack while I waited for the test results. This expression, alluding to a medieval instrument of torture to which the victim was fastened and stretched, has been used figuratively since the late 1500s. Shakespeare had it in The Merchant of Venice (3:2): “Let me choose, for as I am I live upon the rack.”
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.