The Collaborative International Dictionary of English v.0.48:

Seggar \Seg"gar\, n. [Prov. E. saggard a seggar, seggard a sort
   of riding surtout, contr. fr. safeguard.]
   A case or holder made of fire clay, in which fine pottery is
   inclosed while baking in the kin. [Written also saggar,
   sagger, and segger.] --Ure.
   [1913 Webster]