Jacksie is a slang term used in the UK to refer to one's buttocks or rear end. The word is spelled with the letter "j" instead of "y" in order to reflect the pronunciation of the word with a distinct "j" sound at the beginning. To indicate this sound, the International Phonetic Alphabet shows the word as /ˈdʒæksi/. It is a relatively informal word, and not one commonly found in formal writing, but may be used in more casual contexts to refer to the buttocks.
The word "jacksie" is a colloquial term used in British English to refer to the buttocks or backside. Its etymology can be traced back to the late 19th century. It is believed to be derived from "Jack", a common nickname for a man, combined with the suffix "-sie", which is often used as an endearing or diminutive term. Therefore, "jacksie" can be seen as an affectionate or playful way to refer to someone's posterior.