The origin came from the episode Raisins, where the boys went to the hooters bar to show Stan there was other girls out there besides Wendy. However, it was Butters who had developed a crush on Lexus, who was flirty with him, and saying to him how she wanted to see him again. He soon ended going to Raisins everyday to see Lexus, but he was oblivious that she only acted that way with him for money. He found this out soon though, after he revealed to him they wern't boyfriend and girlfriend.
Some say that Buttus is canon, some say it isn't. Some people say it isn't because Lexus only flirted with Butters for money, but some people may think it's canon for Butters really did like Lexus, and thinking Lexus only confirmed that they wern't a couple, and may of deep down have feelings.