Well, yea in Pokemon but the word grunt is a sound, not a person so that's...weird...
No, in the real world, which is where Pokemon got the word from. A grunt is a sound, but it also is another word for
a person of low ranking or standing, as Tortferngatr said--Pokemon didn't invent that use for the term; it's been around much longer than Pokemon has. You may not have heard the term used by itself, but you've probably heard of a person being given a menial task and referring to it as "
grunt work" as some point or another. As for why they picked grunt... well, that's probably because it does have that connotation of "grunt work" to it, which would typically be the first thing you think of when you hear the word--they're on the bottom level, not all that important, easily replaced, there are lots of them, they're not very skilled, and they do the grunt work for the organization. Combine that with the stuff the grunts tend to say, and it's really fitting, which is most likely why they went for it.
Edit: Beaten.