The Collaborative International Dictionary of English v.0.48:

Signpost \Sign"post`\, n.
   A post on which a sign hangs, or on which papers are placed
   to give public notice of anything.
   [1913 Webster] Sik

WordNet (r) 3.0 (2006):

signpost
    n 1: a post bearing a sign that gives directions or shows the
         way [syn: signpost, guidepost]
    v 1: mark with a signpost, as of a path

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "signpost":
   Samson post, arrow, baluster, balustrade, banister, blaze,
   compass needle, direction, direction post, doorjamb, doorpost,
   finger post, fist, gatepost, guide, guideboard, guidepost, hand,
   hitching post, hour hand, index, index finger, jamb, king post,
   lead, lubber line, milepost, minute hand, mullion, needle, pointer,
   post, signboard, snubbing post, stanchion, standard, stile,
   upright