It's a more excited or enthusiastic way of saying "Alright" or "'Ight". It is actually just I YEET without the space, and since yeet means to get pumped up or excited about something, you're really just saying you're yeeting.
Boy: "Hey did that girl we were talking to before say she was comin' over?"

Friend: "Ya man. In a couple hours."

Boy: "IYEET!"
by The Great Dark June 27, 2015
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