The one and only thing that all humans have「uniformly in common as a body part」is an UMBILICUS (abdominal navel), since without that, no one was birthed and the person would not be alive. Also meaning of UNDERSIDE OF A CRAB is using the naked abdomen and legs of a service provider with both no-longer-integrated feet wrapped around two different clients like pincers, with the knees bent to resemble a crab's claws and legs, with everyone's arms and legs flailing「in unison, simultaneously」and drops of salty fluids flying every which way.
　Both scabies and body lice (these are different animals) can also colonize the umbilicus and can be found「uniformly in human body meat」the world over, otherwise known as a navel crabs.
　[quote]:
　The microscopic mite is a tiny, eight-legged creature with a round body. The mite burrows between the skin layers. The body develops a reaction to the mite that results in severe itching. Often intense enough to keep sufferers awake at night, and frequently leading to skin infections.
　Human scabies is almost always caught from another person, anyone who has come into close contact. It could be from a child, a friend, or another family member. Some people do react more severely than others do, and a rare infected person may hardly itch at all.
　Attracted to warmth and odor, the female mite is drawn to a new host, making a burrow, laying eggs and producing secretions that cause an allergic reaction. Larvae hatch from the eggs and travel to the skin surface, lying in shallow pockets where they will develop into adult mites. It may be four to six weeks before a newly infected person will notice the itching or swelling that can indicate the presence of scabies. A re-infection will itch right away.
　The earliest and most common symptom of scabies is itching, particularly at night. An early scabies rash will show up as little red bumps, like hives, tiny bites or pimples. In more advanced cases, the skin may be crusty or scaly. Scabies will usually begin in the folds and crevices of the body -- particularly between the fingers, under the arms, on the wrists, buttocks or belt line, around the nipples for women and on the penis for men. Mites also tend to hide in, or on, the skin under rings, bracelets or watchbands or under the nails. The head and face are not affected, except in children or those with weak immune systems.
　Once diagnosed it is essential that the treatment is properly completed. Thoroughly massage Elimite Cream into all skin surfaces from head to the soles of the feet. It is critical to apply this on every square inch of the body, not only where the rash is. Apply between the finger and toe creases, in the folds of the wrist and waistline, in the cleft of the buttocks and on the genitals and in the belly button.
　[close quote. Https://www.aocd.org/?page=Scabies 2014-06-30　]　

Modern definitions (that generally disregard history) …excluding politically incorrect concepts and other meanings deemed offensive today; may list only pigeonholed definitions, euphemisms, or meaningless mnemonics)
Chinese: navel
Japanese: navel, belly button
Unihan extended: abdominal area of crab; navel
EDRDG: navel