Hi Peter. Hm... I was intrigued by this dirty dozen... Here what I found " The “Dirty Dozen” are watches produced by the 12 British Ministry of Defence contractors during World War II who were tasked with strapping watches to the wrists of allied forces. " https://timeandtidewatches.com/which-of-the-dirty-dozen-survive-today/. I imagine you have checked such issues. So I'm not sure this is the correct context that made you asking what this means. Moreover, I suspect that the watches have this name because of something preexisting in the language, about which I don't know. You should know better, I suppose. Dirty dozen. Why not durty ten... Maybe this is related to the difficulties caused to some cultures by the system SI or by its predecessor meter/gram/second? Interesting to know.