something that is very important or serious.

Don't disturb me unless it's matter of life and death.

• fall off perch
• fall off the map
• fall off the wagon
• fall off your perch
• fall on deaf ears
• fall on hard times
• fall on stony ground
• fall out with
• fall out with someone
• fall over
• fall over yourself to do something
• fall prey to
• fall prey to someone
• fall prey to something
• fall short
• fall short of
• fall short of something
• falls on deaf ears
• fall through
• fall through the cracks
• fall through the floor
• fall to pieces
• false alarm
• false dawn
• false economy