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.