also: あたまのくろいねずみ
workplace thief; embezzler; untrustworthy person
Idiomatic expression literally meaning 'a rat with a black head', used to describe someone who steals money from their workplace or is generally untrustworthy. Often used in a derogatory or cautionary sense.
There's a rumor that the guy in accounting is an embezzler.
I can't forgive a workplace thief who skims company money.