From Wiktionary, the free dictionary

From side +‎ post.

sidepost (plural sideposts)

1. A vertical supporting post beside a door, gate, etc.
   • 2001, A. J. Youngson, The Companion Guide to Edinburgh and the Borders, page 352:

     The gate itself has sideposts of rustic stonework, each topped by a large Baroque vase; these sideposts are joined by elaborate floral ironwork with a coat of arms and a coronet in the centre […]

• deposits, side pots, topsides