The term "back sides" can be replaced by a variety of synonyms depending on the context in which it is being used. Some common alternatives include "hindquarters," "rear end," "buttocks," "behind," "derriere," "posterior," "booty," "bum," "tush," and "backside." While some of these words may be considered more formal or sophisticated than others, they all refer to the same general area of the body and can be used interchangeably in many situations. It is important to choose the right synonym based on the tone and style of the writing or conversation.