A person from Yokohama
Hama is from the "hama" in "Yokohama", which means "shore" in English. "ko" means child. This expression is not really considered to be slang
or anything. The word hamakko is used in the same way the word edokko is used.
There is truly a distinct "hamakko" personality, character, and sense of humor that is its own and is different from all other areas of Japan. Not snobbish or vain like Tokyoites and not quick to speak and loud like Osakans. Maybe what makes
them this way is because they know they are number 3 in Japan and don't have to put up with or care about the constant rivalry such as between the Tokyo east and Osaka west.