I assure you... I am NO NATIVE Dumbfuckistani... nor is my wife. My children however.... will have to live with that curse for the rest of their lives.
But hopefully, we'll be elsewhere before they become irreversibly contaminated.
I am quite conversant in 'merkin English, however... therefore my question about this usage of homely.
Over here, the similar colloquialism is "dumb as a bag of hammers" or "about as sharp as a sack of wet mash".
Being both a machinist and a mechanic, of course I know what a spanner is; in fact, we use it to describe a specific TYPE of wrench which uses a hook or peg as the torque transfer point. It is my understanding that Brits use it to describe a type of adjustable wrench we call a Crescent wrench, after the brand that made them a household item in the same way we call all locking pliers "Vise-Grips".
mnem