To replace words in a sentence, phrase, or song with other words that mean the same thing. Implies poor results, or dumbing down. This is a common practice among hymnal editors of some denominations, who are apparently among the most clueless humans walking the earth.
There used to be an internal rhyme in the bridge, until the publishers thesaurusized it.
Taking a sentence from a (generally) published source and using a thesaurus to change the words so as to avoid plagiarism.
I used one of my earlier papers for this assignment, but thesaurusized it so no one would notice.