Chugger is a combination of the words charity and mugger. A charity mugger is a generally a student or young adult (paid) volunteer who aggressively asks you to fill in a questionnaire for their particular cause be it Concern, Greenpeace, Baby Seal Clubbers. Generally chuggers don't take no for an answer so it is better to never make eye contact with them and tell them to feck off if they even open their mouths. In the movie Airplane you can see an example of how to deal with chuggers effectively.
