area around the navel; treasure trail; sagittal hair
Refers to the region surrounding the navel, often in the context of body hair (the line of hair from the navel downward). 'Treasure trail' and 'sagittal hair' are specific to the hair pattern.
He is self-conscious about the hair around his navel.
When wearing a swimsuit, I worry about my treasure trail.
Compound of へそ (navel) and まわり (around, circumference). The exact historical derivation is straightforward; the word is a transparent combination of its parts.