The Rifleman’s equivalent of the Wayne’s Word “not”, although pre-dating it by about ten years. Added to any phrase it reverses the meaning, thus: “That idea’s good, durch” means it is a potentially poor decision. “That idea’s good durch, sir” means “don’t be so fucking stupid you gormless Rodney”.
Durch seems to have originated from 3 RGJ when they were based in Celle in the 1980s. Apparently it had something to do with a road sign on the way to the ferry ports, durch being German for “through”, although why this should have led to one of the most obscure pieces of slang in the British Army is unclear. The other possibility is some connection with the Audi slogan “Vorsprung durch Technik”; again: why, who knows? The Cowboys were always pretty odd, sort of an East End gangland crime family with APCs instead of Jags. For all I know it could be Cockney Rhyming Slang.
Durch permeated slowly through the rest of the Light Division and subsequently the Rifles, although it has always been more universal amongst 3 RGJ / 4 Rifles. Everywhere else it is most widely used in its most pure form, as the expression meaning “absolutely top notch”: Wank Durch.