booty
Synonym discussion
Sense: noun/2
For the body-part meaning, 'booty' is very informal and playful, especially in songs, jokes, and casual speech. 'Bottom' is the safest everyday choice and works well with children or in polite conversation. 'Butt' is also common, but it sounds blunter. 'Backside' is softer and a little old-fashioned, while 'rear' is the most formal and often appears in notices or instructions. 'Buttocks' is the anatomical word, so it sounds clinical rather than playful. You might say 'The child fell on her bottom,' 'He sat on his butt,' or 'The nurse checked the skin on his buttocks.' Use 'booty' only when a light, joking tone fits.
Etymology
The older sense of 'booty' entered English in the late Middle Ages from a Low German word for goods taken as a share after fighting or raiding. That is why the earliest English uses are about plunder and stolen property. Much later, especially in American slang, the word was extended jokingly to mean a person's buttocks, probably because it came to suggest something attractive or desirable. The informal sense for a valuable haul or a pile of good things also grows out of that older idea of spoils or prize goods.