Private dick, though it may sound dirty, or even redundant, it really isn't as bad as it sounds. A 'private dick' is merely a modern term for the classic
private eye, which is just another nickname for a
private detective. I don't know why private detectives deserve so many
nicknames, but it seems that they've gotten this one because of their reputation for being...well...
not-nice people. Also, they kinda poke around where they're not supposed to be.
Sounds rather fitting to me.