I've been meaning to do this for a week or so...

[dira]

Tonight, to satisfy my own tip-of-the-brain niggling curiosity, I looked up the word for what kind of metaphor you're using when you use the whole thing to refer to a part (technical linguistic formulation: WHOLE FOR THE PART), as when you say, "He's hard," but actually mean to indicate, "His penis is hard."

It's metonymy. The textbook resorted to really flimsy geographical examples in illustration of WHOLE FOR THE PART, presumably to avoid using the one I did.

PART FOR THE WHOLE, incidentally, is more common in language in general, and although it's a type of metonymy, it has its own special name: synecdoche. Unfortunately it's after one in the morning and I watched four hours of Due South today and then went to a Hip show, so I can't think of an example right now.

Edited next morning to add: fairmer helpfully pointed out that when you say "cunt" but mean to indicate "woman," you are using a synecdoche and enacting some interesting gender politics in contrast to the above usage.

Another type of metonymy, OBJECT FOR MATERIAL CONSTITUTING THAT OBJECT, slightly further down the page, yields quite possibly my favorite linguistic example sentence ever: "There was cat all over the road."