"the true skin," 1830, apparently a Latinized form of Greek derma "skin, leather" (see derma); or perhaps a back-formation from epidermis.
Entries linking to dermis
derma n.
"the true skin, the skin beneath the epidermis," 1706, from Modern Latin derma, from Greek derma (genitive dermatos) "(flayed) skin, leather," from PIE root *der- "to split, flay, peel," with derivatives referring to skin and leather.