nuko

One root, no seams. Nuko is nuk- and nothing else. It does not decompose, it takes no prefix in ordinary use, and the only things built on it are transparent compounds like nukdoloro. That is worth saying out loud because the shape invites a wrong cut.

The false seam is nuklea. Nuklea, nuclear, sits 231 ranks earlier and looks like a cousin. It is not one. Nuklea comes from Latin nucleus, a little nut, from nux; nuko comes from Arabic by way of Latin nucha. Two languages, two meanings, one accidental resemblance in the first three letters. Nothing in a nuclear reactor has a neck.

Where it sits on the body. Kolo is the whole neck, front and back. Nuko is the back of it, and the Reta Vortaro stretches the word up over the occiput as well, so it can cover the back of the head where the skull meets the neck. English nape is narrower than that.

The idiom worth knowing. Esperanto Bible translations render stiff-necked as malmolnuka or rigidnuka, a hard or stiff nape, for the person too proud to bend his neck in obedience. That is the same picture English uses and the same one obstina draws on. If you meet rigidnuka in older prose, it is an insult, not a diagnosis.