The term 'Rotten Fellow' can be broken down into three crossword solutions: CAD (3 letters), LOUSE (5 letters), and RATFINK (7 letters). CAD is an acronym for a 'Chief Architectural Draftsman, ' which is a professional who creates detailed drawings of buildings. A LOUSE is a small, wingless insect that feeds on the blood of humans and other animals. RATFINK is a derogatory term used to describe someone who is untrustworthy or deceitful.