dissertation (n.)

1610s, "discussion, debate," from Latin dissertationem (nominative dissertatio) "discourse," noun of action from past participle stem of dissertare "debate, argue, examine, harangue," frequentative of disserere "discuss, examine," from dis- "apart" (see dis-) + serere "to join together, put in a row, arrange (words)," from PIE root *ser- (2) "to line up." Sense of "formal, written treatise" is 1650s.