1. Interjection, usu. preceeded by "oh" and or "buh."
2. Imperative, "calm down."
Etymology: "Sibbuh" evolved from the popular usage of "Simma," from "Simma down now!", popularized by the sketch of the same name on Saturday Night Live.
Ex. 1: James: I think we missed the movie.
Josie: Oh sibbuh. That was the last showing!
Ex. 2: Stephen: What the hell! You cut in line!
Mary: Hey, sibbuh! Ok? Just sibbuh.