early 15c., dissemblen, "assume a false seeming; conceal real facts, motives, intentions, etc.; mask the truth about oneself," from Old French dessembler, from Latin dissimulare "make unlike, conceal, disguise," from dis- "completely" (see dis-) + simulare "to make like, imitate, copy, represent," from stem of similis "like, resembling, of the same kind" (see similar). Related: Dissembled; dissembling.
Form altered apparently by influence of resemble, Old French resembler. Earlier was Middle English dissimule, from Old French dissimuler. Transitive meaning "make unlike, disguise" is from c. 1500; that of "give a false impression of" is from 1510s.
To dissemble is to pretend that a thing which is is not: as, to dissemble one's real sentiments. To simulate is to pretend that a thing which is not is: as, to simulate friendship. To dissimulate is to hide the reality or truth of something under a diverse contrary appearance: as, to dissimulate one's poverty by ostentation. To disguise is to put under a false guise, to keep a thing from being recognized by giving it a false appearance: as I cannot disguise from myself the fact. [Century Dictionary]