When someone speaks, acts or performs around others in a
way that assumes the other
people already know the basics. This can be fine for all parties involved, but it can also be considered rude, depending on what the subject matter is and who's receiving it.
A friend invites you over for dinner with his family. During dinner, the relatives start talking endlessly about Mortimer's problems. You keep thinking: "Who the
fuck is Mortimer?" and no one at the
table has the courtesy to explain to you who Mortimer is. The family was speaking "matter of factly" about some
loser named Mortimer.
Snottrah: Do you want to come back over for dinner?
Perphenalia: Nah. Last
time I was there everyone started talking matter-of-factly about some prick named Mortimer and I sat there
like an idiot.
Snottrah: Oh, sorry. Mortimer's my meth addict
uncle. We disowned him a couple of years back for holding himself at gunpoint and carjacking his own car.
Why is the professor talking so matter-of-factly about That's because you missed the first two classes, genius.