Short for four-wheeling. Used very commonly by off-road enthusiasts. Use of this word separates the people who actually take their vehicles off-road from the posers.
Took the truck out wheeling last weekend and had a blast.
Stranger: "Hey you ever take your Landcruiser muddin?"
Truck owner: "Um, no. I go wheeling pretty often however I don't play in the mud"
Stranger: "Wheeling? Huh, never heard of that. I take my Jeep muddin all the time in a field by my parents house, it's pretty hardcore!"
Truck owner: "Sigh... Silly Jeep-ers"