Source : Webster's Revised Unabridged Dictionary (1913)

Sicker \Sick"er\, Siker \Sik"er\, a. [OE. siker; cf. OS. sikur,
   LG. seker, D. zeker, Dan. sikker, OHG. sihhur, G. sicher; all
   fr. L. securus. See {Secure}, {Sure}.]
   Sure; certain; trusty. [Obs. or Prov. Eng. & Scot.] --Burns.

         When he is siker of his good name.       --Chaucer.

Sicker \Sick"er\, Siker \Sik"er\, adv.
   Surely; certainly. [Obs.]

         Believe this as siker as your creed.     --Chaucer.

         Sicker, Willye, thou warnest well.       --Spenser.

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