General Uses of Ūsus, -ūs

As a (fourth declension masculine) noun, ūsus can adopt a variety of closely-related but powerfully particular meanings:

1. Ūsus + genitive typically refers to the use, exercise or enjoyment of something.

ūsus ocūlōrum: eyesight
ūsus pectōrālis : push-ups
ūsus unguentis: the delights of cologne (I highly recommend getting this at TJ Maxx—half-price!)

2. On its own, ūsus can either refer to ‘exercise’ or ‘wear and tear’

Fidēs nōn ad ūsum tendit: the insurance does not cover wear and tear.
Musculōsa ūsū cotidiānō exstitit: she became very buff through daily exercise
(the more straightforward exercitātiō is more common, at least in my experience)

3. It can also reference a ‘habit’ or social ‘custom’

ūsum loquendī populō concessī; scientam mihi reservāvī: I have give up my habit of making speeches to the people, but I have retained my habit of learning (Cicero in old age)
populum auctōritāte suā ad ūsum frūgalitātis vocāvit: by his authority, he brought the people to a habit of moderation (Lycurgus)

Chewing One’s Nails—In Latin

unguēs praerōdō — I chew my nails

or perhaps—

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