I'm quite familiar with the idiomatic phrase of getting on one's nerves, but less so with getting on one's last nerve, though I had a friend who used a variation: getting on one's third nerve. Is one version preferred over the other in different locations? Is third nerve used anywhere?
It is a small difference in meaning, as "last nerve" is a bit stronger than just "nerves".
Getting on one's nerves
Indicates that the person is strongly annoyed by someone/something.
Getting on one's last nerve
Indicates that the person has been so annoyed (maybe over a period of time) that he is on the edge to snap.
Getting on one's third nerve