[Cuttnhorse]

Fillip: 1 a: a blow or gesture made by the sudden forcible straightening of a finger curled up against the thumb b: a short sharp blow.

Hey...that's what dad used to do to my brother and me when we were being stupid...we used to call them knuckle thumps.

[r9etb]

That’s the verb form.... The Brits tend to use it as “a significant and often unexpected development.”