Still unable to poo? (10)

I believe the answer is:
motionless
'still' is the definition.
(synonyms)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for motionless that I've seen before include "Immobile" , "Still, unmoving" , "Perfectly still" , "Inert" , "constipated?" .)
