Used as a combination of the phrase "Yeah Right". Can be used as an affirmation of another's story, condition, conversation...etc. Cab also be used to express disgust or disbelief at a story, condition, conversation...etc. Phrase was first coined in the early 80's.
So I take her home and we are going at it and she stops in the middle and says she can't because her dead Grandma is watching...Yight!!