A root off of the old saying TLDR (Too Long, Didn't Read) or TLDL (Too Long, Didn't Listen). TSDC means Too Sentimential, Didn't Care or Too Serious, Didn't Care.
Used for situations where the other person is obviously really obsessed over something they believe you should strongly care about.
Girl: I love you so much! Please, I've been dreaming about the day we got a chance to be your girlfriend, and our first date and everything after that..
Boy: I just wanna bone m'kay? 'Cus really, TSDC.