A slap on the skull done by a single or double finger. Usually without momentum, but by sheer tension of tendons and muscles. Either the middle finger is strained against the thumb, and then the thumb is suddenly released and the skull gets the result, or the whole palm is placed over the skull, middle finger is then pulled up with the other hand (like a string of a bow), against its muscles, as far as it would go, and then abruptly released.
Sounds simple, but it can hurt. This was used as punishment in several games or when betting, the loser gets it.
A honorary and largely symbolic one is awarded to anyone who just had a haircut or a new item of clothing or other big thing. This is somehow "for good luck", whatever that was supposed to mean. One guess is that it may have been to balance the odds - if you have something good happen to you, something bad is surely coming, so let's do that right away, small and not hurting much, so the big bad thing may not come, considering the balance already restored.
5-II-2021 - 16-V-2024