Can someone explain this? All I can think of is that Hopper/Walken scene from True Romance, and I suspect that’s not the “joke” here…
In the emoji world, the eggplant emoji is often used to depict a penis due to the vague similarities in shape. A slang term for penis is dick, so this shirt means “Don’t be a Dick”. This phrase is known as Wheaton’s Law.
Here’s a shorter version.
[youtube=t3ICRoe-QIw][/youtube]
Yeah, but the quality is shite.
Well not to fall into the category they are mentioning, but it should technically read Don’t be a"N". Unless of course you’re supposed to say the actual word you’re intending, not the word of the picture. I cant keep up with the kids these days. 
Thanks all! I’m not a gamer (obviously), but I wholeheartedly approve of the concept.