The Collaborative International Dictionary of English v.0.48:

Sickerly \Sick"er*ly\, Sikerly \Sik"er*ly\, adv.
   Surely; securely. [Obs.]
   [1913 Webster]

         But sikerly, withouten any fable.        --Chaucer.
   [1913 Webster] Sickerness

The Collaborative International Dictionary of English v.0.48:

Siker \Sik"er\, a. & adv., Sikerly \Sik"er*ly\, adv., Sikerness
\Sik"er*ness\, n., etc.
   See 2d Sicker, Sickerly, etc. [Obs.]
   [1913 Webster]