One - Can be used as 'goodbye' or 'later'. Short for "One Love".

Dumb - Below normal intelligence.

Man - 2) generally a person who will do incredibly stupid things for a piece of ass.

All taken from Urban Dictionary.

So, 1dumMan basically means:

"Goodbye you stupid person who will do stupid things for a piece of ass."

Alternatively you can just pick out what you would like "one dumb man" to mean from the list:

http://www.urbandictionary.com/defin...m=one+dumb+man

So...