The etymology of abdomen is uncertain. The traditional etymology that is offered by the Online Etymological Dictionary is that it derives from Latin abdere, meaning “conceal”. It is also proposed that the word originally may have meant “belly fat”.
The term abdomen is clearly of Latin origin and appears likely therefore to relate to specific anatomical features as would have been observed by Roman anatomists. Note that the abdomen has two readily observable features: (1) the body fat that covers the intestines and (2) the belly button. So it seems probable that the term abdomen refers to one of those features.
The Latin word abdere, the proposed root of abdomen, is said to derive from ab (“away”) + do (“to give or offer”). The claim is that the combination abdo meant “to put away” and therefore “to hide”. That may be possible if do, or perhaps more accurately doe, actually meant “to put or place” rather than or in addition to “to give or offer”. Thus the proposed meaning for abdo would be “to conceal” and abdomen would be “concealment produced out of related”.
Another possibility is that the word developed as ab dome n, translating as “separation cover related”. Note that a dome is a type of covering and that do translates very reasonably as “separating barrier above” (and hence “cover”).
While both explanations sound very reasonable, they do present a few problems. As stated before, abdomen is a technical term of Latin origin; if it originally meant “belly fat”, one must accept the conclusion that trained anatomists, one of whom obviously coined the term, allowed the meaning of the term abdomen to change over time from a reference to belly fat to a reference to the location of the belly fat. While that is certainly possible, it seems somewhat unlikely given the nature of those who used the term.
Another problem with the second translation has to do with the true meaning of the ab prefix, which is commonly translated as "away". There are two Greek/Latin prefixes that express the idea of “separate” or “separation”: ab and dis. Many scholars treat them as synonyms; however, the two prefixes in fact have two distinct meanings. Ab refers to spatial separation and thus also “away” and “to leave or cease to be present” (evident in such words as abort, abstain, absent, abolish, abdicate)1. In contrast, dis refers to division/separation of a whole resulting from the presence of a barrier. While dis (“separation resulting from a barrier”) clearly has meaning with respect to the word abdomen, one must wonder what spatial separation ab refers to. The ab prefix appears both redundant and inaccurate since the dis/de/do-based root appears to already convey the intended meaning (i.e., existence of a barrier layer). Is it possible that those who coined the word simply were sloppy?
And regardless of which of the two previous translations one might judge to be correct, one must also wonder why the word abdomen did not include a specific reference to (i.e., the original Latin word for) fat. Clearly if the original anatomist who coined the word was originally referring to belly fat, one would think that he would have said “layer of fat” to refer to the layer of fat rather than refer simply to a “separation barrier” or layer of some unspecified material. More so, if such meaning is correct, there appears little within the word to differentiate the fat layer of the belly from, say, the fat layer of the gluteus. Thus, as a scientific term, abdomen appears to be rather poorly conceived.
However, there is another alternative to the traditional etymology: the anatomist who coined the term abdomen may not have been referencing “belly fat” at all; instead, the anatomist may have been referencing the location of the belly button. Here the meaning of ab (“separation surface above” or “surface separation”) has a very clear and relevant meaning (which also perhaps explains why ab appears as a prefix) that would have been readily understood by other anatomists. As for domen, while it still might refer to the layer of body fat below the belly button, it makes more sense that it would have served to clarify the prefix ab. This is, in fact, a point that merits being underscored: the so-called Latin "roots" are often not root words (i.e., core meanings expressed by a commonly used word) so much as Olin-based clarifications for the suffixes that they are attached to.
Of course, there is also the common English synonym for abdomen: belly. While it is quite possible that the English word belly has little if anything to do with the Latin word abdomen, it still could be worthwhile to take a look at its original meaning(s). According to the OED, the word belly originally referred to purses made of leather but took a 90% change in meaning to mean “swell out”. The word then took another radical turn in meaning, becoming a reference to the abdomen. Curiously each of these meanings can be explained via translations. The word belly can be seen to translate as “coin out of surface, surface within out of” (i.e., “purse”), “round surface out of (flat) surface; surface movement surface within out of” (i.e., “to swell”) and, of greatest importance here, “opening out of surface, surface v within out of” (i.e., “the surface containing the belly button”). Thus, while it is possible to assume that the Latin anatomists and common folk of England were all obsessed with fat old men, it may be that both groups developed words that focused on something very obvious (although the etymologists remained blind to it): the navel.