The true origin of "chayis" is unknown, though some same it is related to Doc Fitz's retarded way of saying "there he is." According to legend, when a student walked into his class late, instead of saying "there he is," Doc Fitz said something along the lines of "chahh-eez-suhh," which we interpret as "chayis sir" today.
Chayis may stand alone, though it is commonly followed by sir, as in: "Oh, chayis sir."
Chayis has taken on a much broader meaning, now referring to almost anything the speaker wants, making it difficult to interpret. "Chayis" is ofter seen as a difficult concept by females.
John: Yo, who's that kid?
Ryan: I don't know, some chayis.
Who invited you, chayis?
Ohhh, chayis sir.