Funny how, in maths class, I always thought of angles as absences–the empty spaces between two lines, or between a line and an axis. But when Roland Barthes mentions obtuseness in his discussion of the third meaning in Image-Music-Text, he says:
The word [obtuse] springs readily to mind and, miracle, when its etymology is unfolded, it already provides us with a theory of the supplementary meaning. Obtusus means that which is blunted, rounded in form. (Image-Music-Text, p.54-55)
It’s not the absence, but the object that would fill the absence. Obtusus, the blunt object. Likewise, the acute: from the Latin acūtus meaning sharpened, acuere, to sharpen, acus, the needle.
The point of an arrow versus the snub nose of a shovel.