From the one time in my life I have been fishing for bluefish, I can add some color to Webster 1913's definition of gaff.

There are bluefish fishing expeditions you can take; we once took one in Ocean City, NJ. Lots of people crammed into a boat, which chugged out into the ocean for a few hours, hunting for the school of bluefish in the middle of the night. When we finally found the school, we could see the water filled with bluefish, their scales flashing in reflection of the moonlight.

Bluefish are pretty big, or at least bigger than the sort of fish I'd seen in rivers. Not so big that a single person can't handle it alone, though the bluefish put up enough of a fight to give us a good workout. However, most people were not strong enough to hoist a bluefish out of the water themselves. Trying to lift a bluefish out of the water may also break the line, even the strong line the tour people provided us with. It's possible to get the bluefish out of the water without help, but it takes more fishing skill than most people on such tours have.

This is where the gaff would come into play. There are probably different types of gaff, but the ones I saw were long (maybe 10 foot) poles with giant fishhooks on the end. These spears were placed on racks on the outside of the boat, just behind the fishermen.

It can take a while to wear down a feisty bluefish's strength. Once it had tired enough to end up on the surface, we would yell "Gaff!" at the top of our lungs. There were guys running around on deck, (I forget their official titles, so I'll just call them gaffers) whose job was to follow the shouts. The gaffer could get pretty busy, so sometimes we'd have to wait and keep shouting "Gaff!", hoping the fish wouldn't get away before the gaffer arrived. The gaffer would take the gaff off of the rack, and try to stab the fish with the business end of the gaff, while the fish was flopping around wildly near the surface. Once he had speared the fish, he'd help us hoist it onto the deck. We would thank him for his help, and he would run off, gaff in hand, to help the next person.

That's what a gaff is.

An extra tidbit about the gaffers on our tour. Gaffing was not their only job. They would also, if you wanted them to, clean (cut up etc) your fish for you, in exchange for an equal amount of your catch. (i.e. half) It may sound like a ripoff, but not when you've caught more fish than you can eat before it would go bad. The gaffers would have plenty of time to do that on the trip back to shore.