The Collaborative International Dictionary of English v.0.48:

King-post \King"-post`\, n. (Carp.)
   A member of a common form of truss, as a roof truss. It is
   strictly a tie, intended to prevent the sagging of the
   tiebeam in the middle. If there are struts, supporting the
   main rafters, they often bear upon the foot of the king-post.
   Called also crown-post.
   [1913 Webster]