"positive unbelief, mental rejection of a statement or assertion for which credence is demanded," 1670s; see dis- + belief. A Latin-Germanic hybrid.
Disbelief is more commonly used to express an active mental opposition which does not imply a blameworthy disregard of evidence. Unbelief may be a simple failure to believe from lack of evidence or knowledge; but its theological use has given it also the force of wilful opposition to the truth. [Century Dictionary, 1897]