I must admit that I googled this and this response was so amusing that I copied it for your pleasure; here goes---
Interestingly, the phrase used to refer to pleasant chitchat. This may have started from the "nature of family conversation around the Thanksgiving dinner table." You know, everybody's happy, full, and on their best behavior.
As is often the case, the definition changed over time. However, the circumstances are open to debate. The Phrase Finder believes it had to do with the actual sounds a turkey makes. "Turkey gobbling was a distinct, natural sound on frontier farms." So, to "talk turkey" meant to skip the pleasantries and get to what's important, because farmers are a busy bunch.
Another theory exists, though it reads like more of a joke than documented fact. Way back when, a settler and a Native American went hunting for birds, and caught an equal number of turkeys and buzzards. When the pilgrim "divided the game, he took the two turkeys, leaving the buzzards for his companion." Justifiably annoyed, the Native American responded, "Stop talking birds, let's talk turkey."
Now, how about going to see a man about a dog?