M. E. Kabay has a paper in ACM Ubiquity about viruses, malaware and scumware, Some Notes on Malware. It is a long piece (about 21 printed pages) that introduces the subject of viruses, gives a history, and ends up with a discussion of “scumware.” What is scumware, you ask?
Scumware is any software that significantly changes the appearance and functions of Web pages without permission of Webmasters or copyright holders. For example, a number of products overlay banner advertisements with other ads, sometimes for competing products. Scumware may add unauthorized hyperlinks to a user’s view of a Web page – sometimes using links to possibly objectionable sites. Such programs can interfere with existing hyperlinks by adding other destinations to the intended target. In addition, some products install themselves without warning users of these functions; others bury the details of their Web-page modifications in the extensive legalise of end-user license agreements.
I guess the idea is that such software puts scum on the clear surface of your web pond.