Source : Webster's Revised Unabridged Dictionary (1913)

Twill \Twill\, v. i. [imp. & p. p. {Twilled}; p. pr. & vb. n.
   {Twilling}.] [Scotch tweel; probably from LG. twillen to make
   double, from twi- two; akin to AS. twi-, E. twi- in twilight.
   See {Twice}, and cf. {Tweed}, {Tweel}.]
   To weave, as cloth, so as to produce the appearance of
   diagonal lines or ribs on the surface.