navy-strength
From The Oxford Companion to Spirits & Cocktails
commonly indicates a gin or rum of 57 percent ABV, although in fact the British Royal Navy actually issued its spirits at 54.5 percent ABV. (The misconception arises from the fact that 57 percent ABV is 50 percent alcohol by weight, the old British government-certified strength for proof spirit.) See gin; proof;, rum.
This definition is from The Oxford Companion to Spirits & Cocktails, edited by David Wondrich (Editor-in-Chief) and Noah Rothbaum (Associate Editor).