Someone might say “IHY” when they want to say “I hate you.” It’s usually en expression of anger or frustration. Most likely, the person doesn’t actually hate you. And in some cases, they might even say “IHY” as a way of being playful.
For example, if they’re being playful, it would be in response to a message that was funny. Saying, “OMG, that’s so funny. IHY!”