The harder it is to say the "no" part, the longer you stretch out the "yeah" part.
"Are those budget reports ready?"
"Did you get laid this weekend?"
1. Can be a harmless sarcastic response to a silly question or statement for emphasis. Never meant to be bitter.
2. Said in retort to an incredibly dumb question, no extra wording, but unlike in 1, the question is borderline idiotic or stupid instead of just silly. The response is said with a scathing tone. In internet forums the tone can be (usually NEEDS to be) replaced using elipses (...) or smilies.
3. Said in emphasized agreement or (once again) sarcastic response to a comment (the sarcasm is usually applied when the question is a burn).
4. An emphasis to a statement or set of statements to show just how... ANYTHING an implied situation is (usually the implied situation is bad and will result in failure if ever attempted).
Person B: Lol, yeahno, sorry buddy.
(2.) N00b: i cn sendz dizz 2 bot 4 mod helpz?
Chat Member: .....Yeahno. |:|
(3.) Guy 1: Hey do you think Johnson's gonna land that chick for Prom?
Guy 2: BWAHAHAHA, YEAHNO.
(4.) So, you go into the dungeons of this castle, and you come across this sign. It says "If you go into this cavern you will face a gigantic fuck-off dragon that breathes liquid magma". Then you're given the choice to either advance or retreat. With the description I got? I'm thinkin' "YEAHNO!"