sorry, I'm doing chem homework
An outright lie told by someone wishing to avoid the company of another or hide what it is they are doing from said person.
Guy 1: Hey, wanna skype?
Guy 2: (skyping guy 1's ex/cheating on guy 1/slightly hates guy 1) Oh, sorry, I'm doing chem homework.
Guy 2: (skyping guy 1's ex/cheating on guy 1/slightly hates guy 1) Oh, sorry, I'm doing chem homework.