non-disjunction n.

a prefix used freely in English and meaning "not, lack of," or "sham," giving a negative sense to any word, 14c., from Anglo-French noun-, from Old French non-, from Latin non "not, by no means, not at all, not a," from Old Latin noenum "not one" (*ne oinom, from PIE root *ne- "not" + PIE root *oi-no- "one, unique"). In some cases perhaps from Middle English non "not" (adj.), from Old English nan (see not). "It differs from un- in that it denotes mere negation or absence of the thing or quality, while un- often denotes the opposite of the thing or quality" [Century Dictionary].

c. 1400, disjunccioun, "fracture" (of a bone), from Old French disjunction (13c.) and directly from Latin disiunctionem (nominative disiunctio) "separation," noun of action from past-participle stem of disiungere, from dis- (see dis-) + iungere "to join together," from nasalized form of PIE root *yeug- "to join."