The story
From fat + bike, from the fat tires such bicycles are equipped with, and their correspondingly wider frames needed to accommodate the width of the tires.
Etymology from Wiktionary, CC BY-SA 4.0.
fat bike
fat·bikeToday it means: A bicycle equipped with larger than normal tyres and a wider frame to accommodate them, which provides better traction on off-road surfaces such as sand or snow. Full definition →