Short for thumbed meaning thumbs down, this is the most condescending insult imaginable which dates back to Roman times. thmd is often referred to as a pride-obliterating bitch slap.
A: "yohow do you beat the first level of tetris."
B: "thmd."
This is the accronym you accidentally think is an accronym, but is really an abbreviated version of thank you. If you are like me, you were texted this and spent 20 minutes searching google to try to crack the code so that you would seem cool and trendy to whoever sent it to you. And then you stumbled upon this definition, and now you're both relieved and embarrassed that it took you this long to figure that out.