1. slang A useless drunkard. Sometimes hyphenated. This part of town is filled with addicts and rumdums. That old rum-dum hasn't held down a job in over 10 years. He's been too busy roaming from bar to bar.
2. A very stupid, bumbling, incompetent person. Sometimes hyphenated. I can't believe they appointed that rumdum to run the company. This device is so simple that even rum-dum like me can use it.