Etymonline can help here.
late 13c., misaventure, from O.Fr. mesaventure (12c.) "accident, mishap," from mesavenir "to turn out badly;" see mis- (2) + adventure (n.).
in mischief, miscreant, etc., represents O.Fr. mes- "bad, badly, wrong, wrongly," from V.L. minus-, from L. minus "less", which was not used as a prefix.
c.1200, auenture "that which happens by chance, fortune, luck," from Old French aventure (11c.) "chance, accident, occurrence, event, happening," from Latin adventura (res) "(a thing) about to happen," from adventurus, future participle of advenire "to come to, reach, arrive at," from ad- "to" + venire "to come".
Thus misadventure means "by accident having done something which turned out badly". It's a remarkably concise and exact word and ideally suited to coroners' verdicts.
The -adventure part of misadventure doesn't mean the Boy's Own stuff of an "exciting undertaking", also mentioned by Etymonline. That meaning took three hundred years to evolve from the earlier use of adventure mentioned above.
Meaning developed through "risk/danger" (a trial of one's chances), c.1300, and "perilous undertaking" (late 14c.) and thence to "a novel or exciting incident" (1560s). Earlier it also meant "a wonder, a miracle; accounts of marvelous things" (13c.). The -d- was restored 15c.-16c. Venture is a 15c. variant.