dern (adj.)youdaoicibaDictYouDict[dern 词源字典]
"secret, hidden" (obsolete), from Old English derne "concealed, secret, dark," from West Germanic *darnjaz (cognates: Old Saxon derni, Old Frisian dern "concealed, dark," Old High German tarni "secret, concealed, veiled").

As a verb, "to conceal," from Old English diernan "to hide." Compare 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." Related to dark (adj.). French ternir "to tarnish, to dull" apparently is a Germanic loan-word. [dern etymology, dern origin, 英语词源]