Shy one
When trying to shit. A poo that asks to come out but then refuses to emerge when you sit on the toilet and relax your hole. Not to be confused with constipation which is often characterized by the lack of urge to sit on the toilet.
Oh noooo, not again! this is the third time I've had to sit on the toilet in an hour and it still wont come out. I hate it when I have a shy one.

or:

"Ashley sweetheart, are you OK, you've been sitting on the toilet for half an hour"?

It's OK mom, It's a shy one, can you bring me a mag to read?