Originally Posted by kiwidude
Ignore words is for the lifetime of keeping Sigil open - as soon as you close Sigil, the ignore words list is lost. I should also add that if you switch dictionaries or make other changes in the Preferences screen, that will also clear the ignore words list.
If you want the words to be remembered, add them to a dicitionary. You could have an SFF dictionary for instance and just switch between them in Preferences.