I could be wrong but I have the impression that the part of the body which is hit/slapped is part of the lexical meaning of the verb "spank"
I meant that it's not part of its denotation.
and according to the dictionaries it is the buttocks (or especially the buttocks).
Yes, you can say that it's
especially the buttocks. As I said, think about the agency (the intention) behind the action.
I understand that the punishment without causing serious harm is a key point in the lexical definition of "spank" (and some dictionaries mention it in their definitions) but in this specific definition of the Collins English Dictionary the punishment is not mentioned and the dictionary puts an accent only on the way of hitting and the body part which is hit/slapped.
Yes, they've missed the idea that punishment and/or discipline is a key part of the connotation. Even when spanking is done sexually, that's still a key part of the meaning. The buttocks are relevant only in that that is the area which is most effectively used as a target of the action. Obviously, if you're going to hit a child violently, that's a relatively safe place to do it. And I presume I don't need to explain why it's also this area that's spanked during sexual activity.
Also, there is a ritual called "birthday spanking" which is not a punishment.
I'm not sure what that is exactly, but if it's what I imagine it is, then yes, it is a kind of playful punishment in a sense.
You can slap someone several times on the front of his/her thighs as a punishment without causing a serious harm, but is it a spanking or is it a slapping?
People will think about the
back of the legs, not the front. As I said, with 'spank' think about the agency, the intention, the
reason behind the action.