Is there explanation for why a guitar neck is called a "neck"? I can understand why a guitar body is called a "body" but not the neck.
The "neck" of a guitar is a body metaphor.
The guitar has a "body" "neck" and "head" -- the thinner "neck" connects the smaller "head" to the larger "body," as in the human body and many animals. A guitar is like a flamingo, or a giraffe -- or a person.
In general see this definition neck:
"A narrow connecting or end part of something" (Oxford Dictionaries).
- the neck of a bottle
- the neck of a waterway
- a neck of land with ocean on either side
- "our neck of the woods," an idiom for "neighborhood"/"area" that emphasizes isolation, like a community along a long stretch (neck) of forest path.
- the neck of a guitar or other instrument
Most stringed instruments have "necks" because of the design of the fretboard. Examples of exceptions include the piano and the harpsichord. Since they have necks as a family of objects, even very short and stocky stringed instruments (such as the hurdy-gurdy) can often be described as having a neck.
Anything that is thin compared to regions it is attached to can be called a neck. The term is used a lot in materials science and various engineering disciplines. In fact, you call it necking when pulling on something makes it thin out as it stretches. If the "thin connector" meaning is too much of a stretch (pun intended) from the anatomical meaning for you, just think of it as a word that has two meanings.
In all likelihood the origin is not English. I can't find any references but I expect that when the first stringed instruments were created the long part that sticks out of the "body" was called the "neck" in the local language.
The word "guitar" itself derives from the Sanskrit "tar" (e.g. "sitar") and so I expect the other parts of the guitar (or similar stringed instruments) derive from Sanskrit -- or other languages like Arabic, since the lute comes from the Moorish "oud". Over time these names may have either been translated into English, or the original word used because there was no English equivalent.
Or possibly just made up based on what they look like. Interestingly the equivalent to guitar neck in Spanish is mástil, which means "mast" in English.
Given that the parts of the lute are similarly named, though, my guess is that the parts of the guitar in English derive from the lute, since that's a much older instrument.