Gin pennant
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The gin pennant (sometimes gin flag or drinking pennant) is a maritime flag. When flown aboard ship, it indicates an open invitation to other ships' officers to come aboard for drinks.
The gin pennant appeared at least as early as the 1940s, and possibly long before. Flying the gin pennant was an invitation (generally aimed at officers) to come aboard for a (free) drink in the ship's wardroom. (Consequently, junior officers would sometimes compete and conspire to fly the gin pennant aboard other ships.) Gin pennants likely originated in, and remain in use today, in the Commonwealth navies.[1][2]