Trail term

Trail term
 

On cattle drives the cook's helper was called 'the cook's louse'. One day the helper's own Mother showed up to visit from back East and heard the cook refer to him as the cook's louse. Seeing she was somewhat shocked, he hastened to tell her, apologetically, "It's just a trail term, Ma'am"

Ay Señora, es nada más que un término en plantilla (?)

No es nada más que un término ...
... de sendero.
... empleado en el sendero.

Wait for a native speaker.

I'd say "sólo es un decir", although this expression might be used slightly differently in other regions. :)

Other options to explain that it's only a nickname:
- Nomás es un apodo.
- Así le decimos.
- Es de cariño.
- No se crea, señora, es broma. (Probably exclusively Mexican.)
- Es puro cotorreo. (Also very colloquial Mexican.) ;)

What Angélica says is right in informal, one-of-a-kind situations. If the term "cook's louse" is systematically used in all similar groups in a region, it's jargon (jerga).

In Argentina -and probably other countries in the SH- it would be "es jerga de arrieros" with "arriero" meaning cattle driver, here meaning a cowboy. The term "arriero" equals muleteer in some other countries.