Vet as in veterinarian, not veteran. These are the people you see in comment sections across social media under videos of animals. They will claim whatever activity the animal is doing in the video is harmful to said animal, ruining the fun for everyone. They have no actual veterinary experience or expertise, and they often just parrot poorly-cited articles they just Googled to make their comment.
"Here's a video of my ferret eating steak!"
"UMMM ACTUALLY FERRETS NEED TO EAT WHITE MEAT ONLY. THAT'S BAD FOR THEM. YOU'RE ABUSING HER."
"Wow, someone's being a backseat vet. Actually, any raw meat is fine for ferrets to eat. I've owned ferrets for 20 years, you don't know what you're talking about."