A "callusing" of the skin on ones lower wrist/forearm. Skin is rough,slightly elevated and generally darker than other areas of skin."mouse pads" are a result from using a mouse for extensive periods of time via gaming,porn,or just general use or a computer mouse.
Damn, I've got mouse pads from all that p0rn I watched last night.
"What is that on your wrist?" "Oh, these?" "Just mouse pads."