Is it rude to give someone a "thumbs up" when not feelng like saying hello and inviting unwanted conversation?

I just do it to do it.  No one wants to converse with me. :P

Well I wouldn't do it because I think it is more of a guy thing but if someone gave me a thumbs up I would think he was just being positive. 

  I think in most cases it can be extremely rude. From a very young age, I was taught that to be polite, we should respond to people by speaking, not by nodding or with hand gestures, etc. Also, it's speaking full words, not "yeah" or "uh huh".  There are obvious exceptions, if in a crowded, noisy bus terminal and you need to acknowledge someone who's trying to communicate with you from far away, a thumbs-up or wave is appropriate. 
~

Nah. The middle finger is a lot worse. 

I personally give a thumbs-up If I see someone who's a long distance away. I don't like to project my voice.