I only get 19,200 English only hits for thumby. I checked the first 10 pages and none of them were actual examples of this meaning.

From how rarely I smudge something with only my thumb, I'm guessing it would come from smudging the cell phone screen while texting since that's the only thing I can think of where people use just their thumbs close to something smudgeable.

Usually my computer screen is "indexy" (I'm taking out a trademark on that right now) because children can't seem to learn to point at the screen without touching it.