Create an exclude dictionary. The exclude dictionary is a text file (like the custom dictionary) that contains entries to override the main dictionary. If you want a particular word to be underlined, add it to the exclude dictionary. This is also in http://www.mvps.org/word/FAQs/General/ExcludeWordFromDic.htm
Make sure to give the exclude dictionary the same name as the main language dictionary it’s associated with, except use the file name extension .exc. For example, the English (United States) dictionary is called Mssp3en.lex, so name the associated exclude dictionary Mssp3en.exc.
Also make sure that the file extension is correct, for example, that .txt isn’t appended to the file name extension to give Mssp3en.exc.txt . This can arise if you have display of file extensions disabled within Windows’ Folder Options.