An answer, spoken facetiously, to any question requiring a numerical answer when the responder; 1) does not know the correct answer, or 2) does not care enough to bother giving a proper answer.
Usually, a response of "six-or-seven" implies apathy or a lack of patience concerning the inquiry/inquirer. It is often used to elicit laughter from - and forge camaraderie with - bystanders while showing blatant disrespect toward the person asking the question.
Spoken immediately in answer and said with authority, "six-or-seven" sounds as if the responder knows - without contemplation - exactly what he's talking about while concurrently, obviously waaaaay off.
It is most effective when used repeatedly over time and in various situations where the correct number is much larger than "six" or "seven" - or - when "six" and "seven" are out of context.
Foreman: "How many pounds can that forklift lift safely?
Union Member: "Sixorseven".
Male cheerleader: "Hey guys, what was the final score of the ballgame?"
Second-string Quarterback: "Six-or-seven."