What is "to doubt not but"

Doubt \Doubt\, v. t.

1. To question or hold questionable; to withhold assent to; to hesitate to believe, or to be inclined not to believe; to withhold confidence from; to distrust; as, I have heard the story, but I doubt the truth of it.

   To admire superior sense, and doubt their own!
   --Pope.

   I doubt not that however changed, you keep So much of what is graceful.
   --Tennyson.

   To doubt not but.

   I do not doubt but I have been to blame.
   --Dryden.

   We doubt not now But every rub is smoothed on our way.
   --Shak.

   Note: That is, we have no doubt to prevent us from believing, etc. (or notwithstanding all that may be said to the contrary) -- but having a preventive sense, after verbs of ``doubting'' and ``denying'' that convey a notion of hindrance.
   --E. A. Abbott.

2. To suspect; to fear; to be apprehensive of. [Obs.]

   Edmond [was a] good man and doubted God.
   --R. of Gloucester.

   I doubt some foul play.
   --Shak.

   That I of doubted danger had no fear.
   --Spenser.

3. To fill with fear; to affright. [Obs.]

   The virtues of the valiant Caratach More doubt me than all Britain.
   --Beau. & Fl.