It's a mix of "What is going on?" and "what are you doing later". This is a Southern term that is used quite often as a greeting in large social gatherings. "What are you thinkin?" would be a normal sentence to describe a greeting and ask what's going on later in the night. For instance, "what are you thinkin" may elicit an answer of "it's dead- let's go somewhere cool".
"What are you thinking?" "Stupid, let's go to another barl".
by Michael J Murphy February 25, 2014