sarcasm
Saying the opposite to mock or tease someone.
Sarcasm is saying the opposite of what you really mean, usually to mock something or express frustration in a cutting way. When your friend spills juice all over the lunch table and you say “Nice going, genius,” you're being sarcastic. You don't actually think they did something smart: you're pointing out their mistake with a sharp edge.
Sarcasm relies on tone of voice and context. The same words can be sincere or sarcastic depending on how you say them. If your team wins and someone says “Great job!” with genuine enthusiasm, that's praise. But if your team loses badly and someone says “Great job” with an eye roll and a certain bite in their voice, that's sarcasm.
People use sarcasm to be funny, to vent annoyance, or sometimes just to sound clever. But sarcasm can easily hurt feelings, especially when the target doesn't realize you're joking. Some people are good at delivering sarcasm with a playful tone that makes everyone laugh. Others use it as a weapon that leaves people feeling small.
Young children often struggle to detect sarcasm because they take words at face value. As you get older, you learn to pick up the vocal cues and facial expressions that signal “I mean the opposite of what I'm saying.” A sarcastic person uses sarcasm frequently, sometimes to the point that others find it tiresome.