Supposably is a real word, but the meaning is different. It is a synonym for "conceivable", whereas supposedly means: "according to what is accepted or believe

Supposably is a real word, but the meaning is different. It is a synonym for "conceivable", whereas supposedly means: "according to what is accepted or believed to be true" see: http://www.dictionary.com/e/supposedly-vs-supposably/ http://www.dictionary.com/e/supposedly-vs-supposably/