Hard-on

The part of 'hard-on' that people get fixated with is the hard part; but what interests me about 'hard-on', qua phrase, is the on part; as if an erection is something we wear, something we drawn onto our otherwise inherently soft flesh. An erection, in other words, is the adoption of a localised exoskeleton, not a stiffening from the inside out but rather a strapping on of external armour.