A "maiden voyage" is the first voyage a ship/plane/train makes. After that the ship is experienced so to speak. Similarly when a woman has sex for the first time (unmarried or not) she is no longer a maiden.
You sometimes hear the expression "he broke her maiden." This can refer to deflowering a virgin.
It can be used in horse racing slang too. Here a "maiden" is a horse that hasn't won a race. If a horse has "broken its maiden", it means it has finally won a race and therefore are not classified as a maiden
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