A gentle dig meaning someone’s not the sharpest, slow to clock what’s going on, or a bit clueless. It’s less nasty than calling someone stupid, so you can sling it in with a grin, though it’s still a dig. Comes from dim meaning not bright, like the lights are on low upstairs.
"He said he’s nipping to Cardiff for the seaside. I went, mate, it’s miles from the coast. He’s a bit dim, bless him."