remove material that is considered improper or offensive (from a book, for example), esp. with the result that it becomes weaker or less effective; to expurgate (a book, for example) prudishly

This is another one of David Crystal’s examples of an eponym, from his book Words, Words, Words. Apparently, it may be pronounced either with a long “o” sound (bō…) or with an “ow” sound (as in “wow”).