A combination of the words GUI (Graphical User Interface) and sewage. GUIage commonly refers to a user interface that is visually appealing, but causes a substantial loss of system performance. GUIage typically sacrifices response time in lieu of a prettier, flashier design. Extensive use of Flash and AJAX usually characterize GUIage.
SugarCRM's new user interface is visually stunning, but the implementation was much faster before the developers added all that GUIage.