There are plenty of synonyms for the phrase "onenight stand" including fling, one-nighter, casual encounter and hook-up. These terms often refer to the act of engaging in sexual activities with a person for one night without any long-term commitment or emotional attachment. Some other synonyms include quickie, night of passion, brief romance, and fleeting affair. While each of these words may have slightly different connotations, they all describe a short-lived sexual encounter. Regardless of the term used, it is important to remember to always prioritize consent and safe sex practices in any sexual encounter.