lockfast

From lock +‎ fast.

lockfast (comparative more lockfast, superlative most lockfast)

1. (Scotland) Fastened or secured with a lock.
   • 1881–1882, Robert Louis Stevenson, Treasure Island, London; Paris: Cassell & Company, published 14 November 1883, →OCLC:

     It was such a scene of confusion as you can hardly fancy. All the lockfast places had been broken open in quest of the chart.