buckle - has in certain eras been quite specifically used in the pudenda context. Tying up your sleeve - thus leaves little to explain.

Wild Mountain Thyme & "If my lassie she'll no come, I'll surely find another to pluck" - depends on your intended outcome, can be interpreted as euphemism.

Shanties were pretty much exclusively sung by men deprived of female company so the subjects were: sex, the captain, sex, the weather, sex, the equipment, sex, and sex. The problemo is that collectors had to bowdlerise because of the mores of the day, and Stan Hugill was specific in "camouflaging" as he put it in his book. Those that were at late night shanty sessions tell of the unexpurgated versions that Stan sang. But not if there were women present. At least not any he could spy, I know of one who claims to have been present, if well hidden.