The term Dutch rudder is a slang expression that refers to a method of male masturbation where one person manipulates the recipients forearm in an up-and-down motion, while the recipient holds their own penis. This term is not a standard or widely recognized term, and its usage may vary.

Someone (or something) that has hidden exceptional characteristics and/or future potential, but currently lacks the final touches that would make them (or it) truly stand out from the crowd.

The phrase is metaphorical and relates to the fact that naturally occurring diamonds are quite ordinary at first glance, and that their [true beauty] as [jewels] is only realized through the cutting and [polishing] process.