Board Thread:Questions and Answers/@comment-99.116.149.244-20150117205558/@comment-121.217.132.232-20150123120525

It is katakana and it says "ダム" (damu) which can mean "dam" or "dumb". According to some random on Yahoo Answers Japan, it has no meaning.

Source: http://detail.chiebukuro.yahoo.co.jp/qa/question_detail/q10101516765]