This term was commonly used during the 1960's and 70's when fatal Air crashes were prevalent.
The most notoriously identified flying coffin at that time was the vintage Douglas DC-3 twin engine transport plane. The late Philippine President Ramon Magsaysay died on board his DC-3 when it crashed in Cebu during the late 50's. Philippine Airlines also had its share of air crashes involving their DC-3 fleet. So did the Philippine Air Force.
After the 2nd World War, Filipino entrepreneurs bought a lot of ex US Army C-47's (DC-3's) and converted them into airline use. Structural fatigue due to its extensive war time use coupled with poor and unreliable maintenance & engine failures earned them the nickname "Flying Coffins".
Now a days you would rarely hear or read about airplanes of that sort. This is due to the high quality of aircraft technology and engineering and maintenance practices of todays aviation industry.
Smaller types of aircraft known for their high rate of fatal accidents were aptly called "Widow Makers". The Beechcraft Baron earned this reputation back in the 70's. Pilots claimed it had design flaws that made the aircraft uncontrollable while flying into inclement weather.
I am glad these flying coffins and widow makers are now a thing of the past in Philippine Aviation. I wouldn't ride on one even if it offered a Budget Fare Promo of "one peso" to any point in the Philippines. No siree! I'd rather take my chances on a rickety ferris wheel.