the '?' would seem to indicate the rendering is of questionable nature. the definition fits the word knurl:

To make knurls, beadings, or ridges (on the edge of a coin, a screw-head, etc.); to mill, to crenate

OTOH, there is a slang term given at UD: nurple
this looks like it was coined just to provide the rhyme (as opposed to curple and hirple?)