décharner

To make gaunt or emaciated, almost always encountered in its past participle décharné describing extreme thinness — a wasted body, a skeletal face, or figuratively a bare, stripped-down landscape or style (un paysage décharné, une prose décharnée).

• La maladie l'avait décharné en quelques mois. — The illness had left him emaciated within a few months.
• Son visage décharné inquiétait tout son entourage. — His gaunt face worried everyone around him.
• L'arbre mort dressait ses branches décharnées vers le ciel. — The dead tree raised its bare, skeletal branches toward the sky.

This is a strong, literary word for extreme emaciation — maigrir (to lose weight) is the everyday verb; reaching for décharné to describe ordinary weight loss will sound alarmingly dramatic.