To remove passages considered obscene or otherwise objectionable from (a book, etc.)
To expunge (objectionable material) from; delete.
To remove erroneous, vulgar, obscene, or otherwise objectionable material from (a book, for example) before publication.
To expurgate is defined as to edit or remove material that is considered to be obscene or objectionable.
When you edit out the parts of a book you consider to be inappropriate, this is an example of a time when you expurgate.