Mis·tas·si·ni (m?s?t?-s??n?), Lake A lake of south-central Quebec, Canada, draining westward into James Bay by way of the Rupert River.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.Mistassini (?m?st??si?n?) n (Placename) Lake Mistassini a lake in E Canada, in N Quebec: the largest lake in the province; drains through the Rupert River into James Bay. Area: 2175 sq km (840 sq miles). Length: about 160 km (100 miles)Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014Mis?tas?si?ni (?m?s t??si ni) n. a lake in E Canada, in Quebec province. 840 sq. mi. (2176 sq. km). Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.