What is "can not choose but"

Choose \Choose\, v. i.

1. To make a selection; to decide.

   They had only to choose between implicit obedience and open rebellion.
   --Prescott.

2. To do otherwise. ``Can I choose but smile?''
   --Pope.

   Can not choose but, must necessarily.

   Thou canst not choose but know who I am.
   --Shak.