Don't look now, but apparently excusitis was submitted to Collins Online English Dictionary back in 2012:

A mediocre person who is suffering from the sickness of a mental disease of MAKING EXCUSES every time that aren't made by the successful person.

As of today, it's approval is still under investigation. I guess it was previously used in a book called The Magic of Thinking Big, a self-help, motivational book by David Schwartz.

There, once again you know more about something than you ever wanted or needed to know. :)