He also had some "saltier" expressions about The Dumb, maybe from his time in the Marines, and a few he cleaned up for me. He was very fond of "when God was handing out brains, so-and-so thought He said trains, and he/she didn't want to go anywhere." He laughed and laughed when I suggested, right before I got married, that this explained markgritter (Mark is very fond of trains). He also used "too dumb to pour pee out of a boot" (which I know was cleaned up from the Marine version) and sometimes "too dumb to pour pee out of a boot with a spigot on the toe and instructions on the heel."
But my personal favorites were "he couldn't find his ass with both hands and a map" or "he couldn't find his ass with a torch and a native guide." When I was maybe 6, I came to him anxiously--I wasn't supposed to have overheard Grandpa fulminating about someone's stupidity quite so vehemently in the first place--and asked if he knew that a British person would think he meant a flashlight and a native guide. He kept an absolutely straight face and decided that he was all right with that. He had another one with an actual flashlight in it, but I'm forgetting it.
I haven't been running into particularly egregious stupidity lately, so I'm not sure why this is at the top of my stack. I think maybe it's because it makes me smile or laugh thinking of my grandpa sighing and rolling his eyes. Also because one of the things I love about Foyle's War is that Christopher Foyle doesn't suffer fools gladly either, and they often let him take the said fools completely to bits sometime in the course of the episode, and some of the things he does when he does that remind me a bit of Grandpa, though I didn't put my finger on it right away.