The Arabic “muakhirah” means something like “the behind”. Funny enough, it shares some roots with the word “muta’akhir” which means “being late” and “akhir” which means “the latest”. It’s polite and used even in some dialects to just refer politely to it. Every dialect though has other specific words that are more vulgar. Some are different in every country. Egyptians would use “Tiz” to refer to it and Moroccans would use the more vulgar “Zok”. Middle Eastern countries also often use “Tiz”.