Salaam. Allaah knows best, and I am confused about it too. I assume you are meaning: "below the navel"?
I tried to look for a hadiith, but couldn`t find any to refer to directly right now except from my memory. I remember I`ve read that the area is referred to `below the navel` by a SoHaabii (I don`t remember which SoHaabii -rAa- and if it was SaHiih). The other authentic Hadiith mentions pubic hair anyway, so the inguinal area is of course the intent. It might be that due to shyness it has been left without detailed explanation, but I hope you will find a satisfactory answer, because The Messenger of Allaah -sAaws- didn`t hide anything that will benefit us.
If you want to be on the safe side, then shave the hair on your belly from under your navel until the genitals and shave the pubic hair too - i.e. what is around the genital between your legs without concerning any part of the hair of the legs. I am of the opinion that the pubic hair is enough, but Allaah knows best.
The thing is, it`s all about purity, if you think about it. People may have differences in the ways that hair grows in that area, but from the medical side the skin contains special sweat glands called the apocrine glands, which empty their secretion to the hair follicles. These type of sweat glands are restricted to the armpits and the pubic area, and they secrete proteins which are not secreted by other sweat glands, and these proteins are used as nutrition by the bacteria on the skin surface, which then causes sweat smell in those areas even though the sweat itself is actually without odor. Okay? So you can appreciate the wisdom of Allaah tabaaraka wa ta`aalaa that He has decreed for us to regularly remove that hair in the armpits and in the pubic area, so that potentially harmful micro-organisms on the skin are cleansed. WALHAMDU LILLAAH. WA SOLLA ALLAAHU ÀLAA NABIYINAA MUHAMMAD WA ÀLAA AALIHII WA SOHBIH.
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