This is one of those little words that is obvious on the surface, but a little tricky to pin down exactly. It’s used a number of times in the suttas alongside other terms to depict land that is impassable or rugged: mountains, thick forest, and the like.
Nadi is “river”, du-(g)ga is “hard to pass”, and vi either means “division, split”, or is simply an intensifier.
Ven Bodhi in AN uses “river hard to cross”. But this sense seems impossible in DN 26, where the various inaccessible places are said to be “entered into” for hiding and protection.
Curiously enough, the commentary varies in almost every explanation.
Nadīvidugganti nadīnaṃ antaradīpādīsu duggamanaṭṭhānaṃ
Nadīvidugga means in inaccessible places of rivers, such as islands etc.
Nadīvidugganti nadiyo ceva duggamaṭṭhānañca
Nadīvidugga means both rivers and inaccessible places
Nadīvidugganti nadiyā viduggaṃ chinnataṭaṭṭhānaṃ
Nadīvidugga means places with banks cut by rivers that are hard to get across
SN 45.11 (the term is found in comm, but is not in sutta)
nadīviduggādīsu setuṃ attharanti
they lay bridges at nadīviduggas etc.
Nadīvidugganti nadīnaṃ bhinnaṭṭhānaṃ taṃ duggamattā nadīvidugganti vuccati
Nadīvidugga means places cut by rivers are hard to get across, so they’re called nadīvidugga.
Nadīvidugganti nadīnaṃ duggamaṭṭhānaṃ antaradīpakaṃ
Nadīvidugga means islands in the middle of rivers that are inaccessible places.
Obviously the general meaning is similar, and the matter is of no great significance. Still, it is important to remember that the commentaries are not omniscient and perfect, but are often more like collections of tips and interpretations.
There does seem to be a general trend, however, that the term refers to the place—whether the cut-up banks and gorges, or islands—rather than the river itself. I suggest “inaccessible riverlands”.