"There you go. A decapitated head."
"That's not decapitated. A decapitated head means a head whose head's been cut off. You mean a severed head."
"I meant what I said."
"Look. 'Decapitate' means 'to remove the head'. From the Latin,. 'De', a prefix implying negation, and 'caput', head. You haven't removed a head from that. It's nothing but a head."
"You're arguing from etymology, there. As a linguist, you should know better."
"Do you want to pass the goddamn course or not? ... Anyway, 'sever' means to cut apart. That's not cut apart, it's whole."
"Rubbish. Something that's been cut away is severed from its parent body. Like that."
"No, no, the neck's been severed, not the head."
"The neck's been severed, resulting in a severed head. Modern usage gives 'sever' some flexibility, as a verb. Now, if you had a severed neck, that'd be a neck in two pieces. A limb, though, that could go either way. Man walks into the ER with a severed arm, he could be clutching at a stump, or he could have a detached arm in his other hand."
"Would you prefer that to be a detached head, then?"
"Don't be silly. It's technically correct, but the connotations are all wrong."
"Don't tell me I'm silly. Look. Usage changes over time, right? So that thing is decapitated."
"Language changes diachronically, yes. But viewed synchronically, a language has a set of production rules at any given time, and if you break them, you're doing it wrong."
"A language is nothing more than a set of mutually intelligible idiolects. And, right now, the dominant idiolect belongs to the guy grading your assignment."
"OK, fine. I'll write about ... hold on ... Isn't that my mother?"