Umbus

Umbilicus est medius locus corporis, dictus quod sit umbus iliorum. Unde et umbo appellatur locus in medio clypei, a quo pendet. ex eo enim infans in utero pendet, ex eo etiam et nutritur. ilium Graeco sermone appellatum, quod ibi nos obvolvamus. Graece enim εἰλύειν obvolvere dicitur. clunes vocatae, quod sint iuxta colum, quod est longao. nates, quod in ipsis innitimur dum sedemus. unde et conglobata est in eis caro, ne prementis corporis mole ossa dolerent.
(Isidorus of Sevilla, Etym. 11.1.99-101)

The navel (umbilicus) is the center of the body, so called, because it is a protuberance (umbus) of the belly. Whence also the place in the middle of a shield is called a ‘boss’ (umbo), from which it hangs, for the infant hangs from it in the uterus and it is also nourished from it. The private parts (ilium, i.e. ilia) are referred to with a Greek word, because there we cover ourselves up, for in Greek ilios (i.e. εἰλύειν) means “cover up.” The buttocks (clunis) are so called, because they are situated next to the colon (colum), that is, to the large intestine (longao). The nates (natis) are so called because we support ourselves (inniti) with them when we are seated. Hence also the flesh is bunched up in them, so that the bones may not hurt from the heavy weight of the body when it presses down. (tr. Stephen A. Barney, W.J. Lewis, J.A. Beach and/or Oliver Berghof)