What’s the connection between ערף meaning "back of the neck" and "drip"?

QUESTION: Can anyone explain to me the root ערף? Why can it mean both “drip/flow” and “back of the neck” (and “breaking the neck” as a presumably denominative verb)? And is there any connection to ערפל (“thick rain clouds”)? Also, is there consensus here that ערף is a metathesis of רעף and/or vice versa? Seems like a no-brainer to me, but I wanted to know what the different opinions are. If you disagree, how did both come to mean “drip”? What is the core meaning of each? Bonus question: what is the precise difference in