Someone who looks blank, clueless, or a bit dozy, like the lights are on but nobody’s home. It’s not always calling them thick, more that they’ve missed the obvious moment. The word’s old, from gorm meaning understanding, so gormless is literally without it. Handy, and pretty satisfying to lob in an argument.
"Fire alarm’s going off and Gaz just stands there gormless, clutching his butty. I’m like, Ey up lad, shift it. He goes, Thought it were nowt."