Keister is a slang word for buttocks and has several synonyms that can be used in both formal and informal language. Some of the common substitutes for keister are bum, derriere, posterior, behind, backside, rear end, tush, and glutes. Tush is often used among younger people and is considered a more playful term. While derriere and posterior are more formal and are commonly used in literature and academic papers. Other terms like bum and rear end are slightly more casual and are used more commonly in everyday language. In summary, There are several synonyms for keister that can be used depending on the context, tone, and audience.