Is it ensure, insure, or assure?
"secret, hidden" (obsolete), from Old English derne "concealed, secret, dark," from West Germanic *darnjaz (cf. Old Saxon derni, Old Frisian dern, Old High German tarni "secret, concealed").
As a verb, "to conceal," from Old English diernan "to hide." Cf. Old High German tarnjan "to conceal, hide;" German Tarnkappe "cloak of invisibility." Related to dark (adj.). French ternir "to tarnish, to dull" is a Germanic loan-word.
(also darned or darnfoolor derned or durned) Wretched; nasty; silly: sentimental songs, darnfool ditties, revival hymnsadverb
: She was darn excitedinterjection
(also darn it or dern it or durn it) An exclamation of disappointment, irritation, frustration, etc: Darn, I've dropped my glockenspiel!
[1780s+; euphemism for damn, which is regarded by some as taboo; probably based on earlier darnation, ''damnation,'' attested by 1798]