Terra firma-forme dermatosis (TFFD), sometimes referred to as Duncan's dirty dermatosis, is a condition characterized by asymptomatic, hyperpigmented, dirtlike plaques that are resistant to regular washing with soap and water but easily rub off with 70% isopropyl alcohol.

TFFD is the result of disordered and delayed keratinization of an unknown etiology. TFFD may appear anywhere on the body but is commonly found on the neck, axilla, and trunk. While it occurs more frequently in children and adolescents, it can be found in adults. There are currently no known predisposing risk factors (including age, race, sex, geography, medications, or comorbidities). Although not a life-threatening or contagious condition, it can lead to unnecessary tests (biopsies, endocrine evaluations, etc) and can be cosmetically bothersome for individuals unable to achieve clean-appearing skin.