Boyd’s Bible Dictionary:

A primitive way of keeping the hull of a ship from opening by passing a cable tightly around it

Concise Bible Dictionary:

The binding of either ropes or chains round a ship in bad weather, to keep it from parting asunder in consequence of its being old or because of its imperfect construction (Acts 27:1717Which 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. (Acts 27:17)). It is called in the navy frapping.

From Manners and Customs of the Bible:

Every ship carried large cables, which were used in case of necessity for passing around the hull, thus “undergirding” it, and saving it from the strain which resulted from the working of the mast in a storm.