On imgur, there is a post with screenshots of an individual's row of Twitter posts. In them, he is telling a story making use of what I (as a non-native speaker of English). I was originally guessing the writing to be in a form of AAVE, but answer so far convincingly make the argument that it's common slang. At one point the author mentions "the valley", perhaps indicating that he's from Silicon Valley. In one of the replies used to continue the story (second one from the bottom) he writes:
Best believe she was pissed.
- am I correct in assuming this to be a device of storytelling?
- if so, is there a technical term for such a device?
- what other subtext, if any, can you as native speakers extract from that sentence in the context established by the story as the author tells it?