1520s, "refusal to grant what is requested or desired;" see deny + -al (2). Replaced earlier denyance (late 15c.). Sense of "act of asserting to the contrary, contradicting" is from 1570s; that of "refusal to accept or acknowledge" is from 1580s. In some 19c. uses, it really means "self-denial." Meaning "unconscious suppression of painful or embarrassing feelings" first attested 1914 in A.A. Brill's translation of Freud's "Psychopathology of Everyday Life"; hence the phrase in denial, popularized 1980s.
in rhetoric, "a denial of an intention to speak of something which nonetheless is hinted at," 1650s, from Late Latin apophasis, from Greek apophasis "denial, negation," from apophanai "to speak off," from apo "off, away from" (see apo-) + phanai "to speak," related to phēmē "voice" (from PIE root *bha- (2) "to speak, tell, say").
early 15c., negacioun, "an act of denial," from Old French negacion (12c.) and directly from Latin negationem (nominative negatio) "denial," noun of action from past-participle stem of negare "deny, say no," from PIE root *ne- "not." As "a negative assertion," mid-15c.
"a repelling; a check, a defeat; peremptory denial or refusal," 1610s, from rebuff (v.), or from French rebuffe or Italian ribuffo.