désobésifier

A rare, modern coinage meaning to make less obese, or to counteract the effects of obésifier ('to make obese') — dictionaries that list it mark it as uncommon and often slightly informal or clinical. A learner is very unlikely to need this outside a specific medical or health-marketing text.

• Le programme vise à désobésifier les patients à risque. — The program aims to reduce obesity in at-risk patients.
• Certains régimes prétendent désobésifier rapidement. — Some diets claim to quickly counteract obesity.
• Les médecins restent prudents face aux méthodes censées désobésifier. — Doctors remain cautious about methods claimed to counteract obesity.

This is a genuine dictionary-attested word, not slang, but it's rare enough that maigrir (to lose weight) or perdre du poids are what a French speaker actually says in ordinary conversation.