- Its constituents have a well-known association. Such as "honey" and this picture
- The meaning (and association) of the compound is unrelated to the associations (and meanings) of its constituents. Or in other words: the open compound noun is not obvious given the two individual words (seems considerably harder than finding closed compound nouns).
- The actual origin of the compound noun is irrelevant: it's about the general association that we have when looking at the words individually. Looking at "honey" individually, we think of honey. Looking at "moon" individually, we think of the moon. So "honeymoon" is the sort of compound noun of which the meaning is different from its constituents (too bad its not open).
- The compound is open/spaced, such as tennis shoe
Note that "tennis shoe" is an example of exactly the opposite of what I'm looking for. Tennis has a certain association. Shoe has a certain association. So the meaning of "tennis shoe" is obvious.
Nota bene: even though my question is closely related to my other question, the answers to my other question were mostly closed compound nouns. This question specifically targets open compound nouns, so it is different both from my previous questions and it needs (satisfactory) answers that were not given in the other question.