If you make this gesture, you’re cocking a snook. I can’t remember the last time someone literally cocked a snook in my direction, though I’d welcome it. It’s such a cheerily childish gesture – the equivalent of telling someone to get lost.
Google cock a snook (I did). There’s no definitive answer to what a snook is and why we might feel like cocking it.
But I’m adding it to the small collection of lovely English words and phrases I’m curating. The collection includes fine fettle, mithering and fretting, mardy and warming the cockles of your heart. Working themes are anxiety, insults and good cheer.