under arms—said of people who are armed and prepared for fighting:
- Japan still had millions of troops under arms and thousands of combat aircraft.
under the gun—acting under a lot of pressure:
- They did a great job, especially working under the gun with only a few months to get it organized and executed.
Note: The expression does not correlate in meaning with the phrase under the knife—used of smb. undergoing a surgical operation:
- She was under the knife for several hours. When she came out of surgery, doctors said she was fine.