Sidepiece is a colloquial slang term used to refer to a person who is not the primary partner in a romantic relationship but rather an extramarital affair. The word has a negative connotation and is often derogatory. There are several synonyms for sidepiece, including mistress, paramour, lover, partner in crime, fling, or affair partner. These words are similarly used to describe a relationship that involves infidelity. It is important to note that the use of these terms is disrespectful and can cause hurt and pain to those involved. It is essential to treat others with respect and dignity, regardless of their relationship status.