"secret, hidden," from Old English derne (West Saxon dierne) "concealed, secret, dark," from West Germanic *darnjaz (source also of Old Saxon derni, Old Frisian dern "concealed, dark," Old High German tarni "secret, concealed, veiled"), related to dark (adj.).
Archaic or poetic only after 16c., it was important and productive in Middle English, with extended senses of "secluded; profound, mysterious; stealthy, deceptive; private, confidential." Dern love was "secret or illicit love; a mistress."
As a verb, meaning "to conceal," it was from Old English diernan "to hide." Compare Old Saxon dernian, Old High German tarnjan "to conceal, hide;" German Tarnkappe, Tarnhelm "magical cap or helmet which turns the wearer invisible or allows him to assume any form." French ternir "to tarnish, to dull" apparently is from Germanic.
an American English variant of darn (interj.), attested from 1830. Alternative form durn is attested by 1835. Related: Derned; durned.