this is not a valid coinage, but is a name i made up that expressed humorously to other kids in primary school (under 10 years old) my experience of doing a poo in the closed cubicle i was in.
the name was "ernie doo".
when you are younger, you tend more to like the feeling of having a full bowel for a while whereas when you get older, it is a nuisance that must be dealt with immediately.
so think of it this way
1. the straining phase
this is when you are trying to push it out because it has hardened somewhat, and if you can think of "errrrrneeee!! !" said like a person who is straining to lift a heavy weight may say it, then you have the idea.
2. the release phase
"doo" was always said with a sigh of relief when the package finally dropped into the water and the kids always gathered around to hear my dramatic vocal expressions and they laughed.
i had one friewnd who was quite disgusting, and he liked to hold his s**ts for a while (we were 9 so don't judge), and when i felt like going to the toilet (poo) i used to mutter "getting a bit of an ernie there?". and he used to roll on the floor with laughter.