A strange-looking word, it comes from obese plus the ending -genic, something tending to generate or create. It refers to conditions that lead people to become excessively fat — a worrying trend in developed countries, especially among young people, who are eating too much of the wrong things and not taking enough exercise. The problem is variously put down to social causes (too many sedentary pursuits available; fear that the outdoors in cities is dangerous, leading to less cycling, walking and running about) or to the results of our consumer lifestyle (eating pre-prepared meals that contain excessive sugar and fats). The term seems to have appeared in the last decade (the first example I can find is from a British newspaper in 1996) and is not as yet mainstream, though it is increasingly turning up in newspapers and medical journals. Its opposite is not often called for, but if you need it, it’s leptogenic, leading to weight loss, from Greek leptos, thin, fine or delicate.