Ship's biscuit, also known as hardtack is a biscuit or cracker made from flour, water, and sometimes salt; it is long-lasting and inexpensive. Although very hard when fresh, it tends to soften a bit over time due to humidity. Also, over time, weevils would infest the biscuits. Small holes are made in the biscuit before baking to allow it to thoroughly dry before packaging.
it was the usual practice to tap the biscuit on the table to induce the weevils to leave the biscuit. The species likely referred to as weevils were most likely the larval stage of the Cadelle Beetle (Tenebroides mauriticus) sometimes referred to as 'boatmen' due to the many legs on each side.
Glossary: SearchWikipedia
Also: