A very urgent issue, situation, or circumstance. This expression can be used either literally, as in She told the doctor to hurry as it was a matter of life and death, or hyperbolically, as in Don't worry about finishing on time—it's hardly a matter of life and death. First recorded in 1849, it alludes to such urgency that someone's life depends on it. Although a matter of life or death would make more sense, it is rarely put that way.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.