At the risk of sounding genuine I'll try to answer this. Keep in mind everyone's definition is different though. And not all magnum shooters, for instance, fall into this category.
To me, a fudd is someone who actively opposes progress in hunting. The vast majority of the time because they think it makes them more of a man and it gives them a superiority complex over other hunters. They don't NEED silly gadgets like suppressors or trekking poles (sissy sticks), and the fact that they don't need that makes them better than the rest of us. Nevermind the fact that we don't need those things either, we just like them and they improve quality of life while hunting/shooting. They conflate the ability to endure hardship, which is a virtue, with needlessly making their hobbies harder than they need to be as a way to prove to themselves and everyone else how tough they are.
If you made a venn diagram of a fudd and people who will reject FBI gel tests in favor of telling you their grandfather stopped a charging "jap" at 100 yards in WW2 with a .45 ACP, it would be a circle. Their masculinity is tied up in all of the choices they make in regards to rifles, gear, etc. So they can't budge from those choices or else they'd be less manly. You'd never get them to admit they flinch when they shoot more than 5 shots in a session with their 300 Remchesterby Shitkicking Magnum (which weighs 7#). And if you tell them they don't need one, you'll get regurgitated BS that their dad told them about Knockdown Power™ passed down from a time when bullet construction sucked.