The phrase "monkey on back" typically refers to a burdensome problem or addiction that is difficult to overcome. There are several synonyms that can be used to describe this situation, such as a weight on one's shoulders, a cross to bear, an albatross around the neck, a thorn in one's side, or a millstone around the neck. Each of these phrases conveys a sense of something that is holding a person back or making their life more difficult. By recognizing these synonyms and using them in conversation, individuals can gain a clearer understanding of the challenges that they and others may be facing.