When a gigantic piece of crap has been lovingly nurtured and has reached its final level of growth and development. At the end of this stage it's evacuated from the safe, warm environment that is the bowels. Coined in the South Park episode "More Crap".
In real life, the turd trimester can be compared to the status of pulling tongues.
As a doctor I cannot condone travelling on an airplane in your turd trimester. It is not safe.