A fling is when you have a short, casual romantic relationship with someone. It's usually nothing serious.
When people talk about having a fling, they're often referring to a brief and not very committed romance. For example, "I had a fling with someone over the summer" might mean that you dated casually for a few months but didn't plan on making it long-term or serious. A lot of times, people have flings when they're traveling or in new places and don't want anything too complicated.
Also, you can use "fling" to talk about throwing or tossing something with a bit of force - like, if you fling your bag onto the bed, it means you're putting it down there fairly roughly.
An act of throwing, often violently.
