The phrase "back of" refers to the area behind something. Some synonyms for "back of" include "rear," "behind," "posterior," "tail end," "hindmost," and "backside." These words are often used interchangeably to describe the position of an object, person, or animal in relation to another. The phrase "at the back of" can also be used to describe the location of something, such as a store or building. It is important to note that the use of synonyms adds variety to language and makes writing more interesting to read.