Meaning

noun the fleshy part of the human body that you sit on

noun external female sex organs

originThe British naval slang sense derives from Fanny Adams. Tins of mutton introduced as rations were not liked by the sailors and were taken by them to contain the butchered remains of Fanny Adams who had been brutally murdered and dismembered. The tins were re-used for eating from and cooking with.

Etymology from Wiktionary, CC BY-SA 3.0