unguēs praerōdō — I chew my nails
digitōs meōs praerōdō — I chew my nails (literally, ‘I gnaw at the tips of my fingers.’
This second one is conjecture. The phrase is sourced in Plautus, Pseudolus, where the image is of guests literally gnawing at their fingers because they are enjoying a feast so mindlessly that they lose track of where the ham ends and the hands begin. That said, ‘chewing the tips of one’s fingers’ could easily fit with the image of gnawing at one’s nails—don’t you think?
There’s nothing in the L&S entry for unguis to settle the case, but here’s the L&S entry for praerōdō—http://goo.gl/pYh68p