This seems to be a common expression in America but it is completely unknown in Ireland. It is used to mean ‘bottom’ and seems to be an informal and inoffensive word often used with children. According to Daniel Cassidy in his crazy piece of garbage How The Irish Invented Slang, this word derives from the Irish word ciste meaning ‘chest, coffer, treasure, fund’. This is nonsense. The Irish word is derived from an old English or Scots expression for a trunk or case (the related word kist is still used in Ulster-Scots) and this has a cognate in German, Kiste, which is pronounced like keister. The German expression Kiste has several meanings. One is trunk or case and the other is what you use to sit on a trunk or case, your backside. This is the origin of the word keister in American speech. As I have already said, the term keister is completely unknown in Irish English, and the word ciste does not have the meaning of backside, so Cassidy’s claim is as stupid and incompetently-researched as everything else in this apology for a book.