Becoming Adolf by Cohen is about the history oh Hitler's toothbrush mustache. The man Cohen grows out a toothbrush mustache of his own. He finds out that only trouble comes from having this mustache and Hitler has ruined the opportunity for anyone else to have it.
Cohen uses his own personal research as well as research from history. When he has the toothbrush mustache, people's attitudes change completely. Some people find it offensive and plain rude to have the mustache because it has been tarnished by Hitler's legacy.
Cohen learned that under no circumstance will anyone accept the Hitler mustache anymore. It is now just a symbol of evil in most peoples eyes and will never be socially accepted. There would be noway for anyone to reclaim this mustache and it will always be last remembered as Hitler's mustache and no one elses.