Indeed. However, what is more likely in this particular situation?
"McCoy, I remember when you told me that in your divorce, your wife left you nothing but your bones, so I'm going to start calling you Bones as a measure of empathy and affection."
- or -
"McCoy, I'm a big fan of ancient military vernacular, and I think you should be given the nickname of Sawbones, which I will shorten to "Bones", because military surgeons of the day, (that being several decades before the Civil War of the United States), used that name."
J.
I don't know much about ancient military vernacular or the US Civil War, but I've certainly heard the term "sawbones" before, and not in the context of Star Trek. Certainly neither of the dictionaries listed by Dictionary.com as carrying
carrying "sawbones" have labelled it as archaic.
I find it very plausible that a surgeon using a nickname associated with surgeons would've been given that nickname because he was a surgeon. I don't find it all plausible that a surgeon using a nickname associated with surgeons would've been given that nickname because he used the word in an anecdote once. Of course it's not exactly an either/or issue, there could've been multiple contributing influences.
Venturing dangerously into interior US affairs here, but isn't McCoy presented as an old-school "southerner"? Wouldn't the use of an outdated nickname be consistent with such a characterisation?