Noah Webster's Dictionary (v. t.) To blind below; to gird round the bottom.

Multi-Version Concordance Undergird (1 Occurrence) Acts 27:17 Which when they had taken up, they used helps, undergirding the ship; and, fearing lest they should fall into the quicksands, strake sail, and so were driven. (Root in KJV WEY WBS YLT NAS RSV)