Superstantial first appeared on The Colbert Report on 9-11-06 as the word next to Stephen during the opening of the show.
While it is likely the portmanteau of 'super' and 'substantial', Stephen Colbert has yet to explain its meaning. It's believed that Jimmy smoked too much weed one day during Colbert's Emmy vacation and changed it without permission.
Fans have attempted to explain that since substantial, contains the prefix "sub," meaning low, superstantial is the opposite, meaning "high, or superior to." The word substantial is actually derived from the word "substance". "Superstantial" can be effectively used in a sentence such as: Stephen Colbert is superstantially more truthy than other news anchors. For instance, Stephen is above the truth. We all need to achieve this manner of thinking in order to see the truth more clearly. Then, you can have an eagle's eye (aka Stephen Jr.'s) view of the truth. That way you can easily pick out the parts that you feel are relevant (hence the truthiness).
For those of us who want to FEEL the meaning of this word instead of reading such a "facty" article, it means information that is ABOVE the truth. We've all found a way to rise above the truth with Stephen's power of truthiness.
Since "super" tends to mean the opposite of "sub", "superstantial" can also be interpreted as the opposite of substantial.
The word "superstantial" appears in Mary Anne Atwood's 19th Century work "A Suggestive Inquiry", in reference to a metaphysical substrate to the physical universe.