While it's not canon, I've always envisioned Arthur's body as being marked with self-inflicted scars, particularly cigarette burns. Little clusters of irregular circles, some silvery and faded, others still-red and raised. All gathered together in little groups, making it obvious he had spent time re-lighting the same cigarette over and over just to put it out on his skin.

I imagine when Arthur first began doing this he tried to keep it from his mother by covering the places he'd burned, hiding his body partially out of shame and partially because he feared her reaction. But eventually he would forget, or she'd walk in on him shirtless, or the sleeve of his jacket would expose his forearm, and she'd see a line of blistered marks. He'd know she had seen because she'd pause in the middle of a sentence and her jaw would tighten just a little, and he'd tense, waiting for her to yell, or cry, or demand he explain himself. But then she'd just continue speaking as if she had not seen anything at all.

So the scars and the blisters would become this unspoken entity; Arthur feeling both relieved and hurt that his mother refuses to acknowledge even the understandable pain of a burn, and his mother treating the visibility of it like roaches passing by on the floor between them.