Situation 1: A word used to veer off of a subject or a response to a question or statement that you don't know how to respond to.
Situation 2: This word could be used to retaliate from a verbal attack in a friendly and joking way.
This was first used as an inside joke when one friends status was in German and the other friend commented "Dah" because he didn't understand the status as a joke. Then, for every other comment he didn't know how to respond to he said "Dah"
Q: "I heard you kissed that ugly person at that party last night."
A: "Uhhhhh Dah?"
Q: "Wow, you are horrible at football."