The meaning of the underlined word "promiscuously" as it is used in the passage is the following one:
Haphazardly.
Explanation:
If the bones lay "promiscuously" upon the earth, it means they did not lay there in a particular order, that is, they were simply thrown there in a completely random way. In other words, the bones lay "haphazardly" upon the earth.
By saying that the two locks of hair conspired to do something, the author is attributing a characteristic of human nature to the locks of hair which on fact are non living (or human) entities
