Coined by Italian-Canadians during the height of Italian immigrated to Canada. Translation to English is "cake eater". There are a few opinions of the direct origin. When Italians first settled into predominantly WASP neighborhoods around Toronto, they were often welcomed by these people of Anglo-Saxon decent with cake (i.e. pound cake). The perception by Italian-Canadians at this point was that Canadians eat a lot of cake, hence the term mangiacake.
Maria: These Canadians, always eat cake.
Vince: Yah, they are what we a call "mangiacake"
Reasons for an Italian-Canadian to call someone a mangiacake:
If you use butter to cook, versus extra virgin olive oil, then you are a mangiacake.
If your pasta sauce is from a jar branded as Ragu, Prego, etc, then you are a mangiacake.
If you pronounce Bruschetta as "broo-sh-edda", you are a mangiacake.