I found nichtentheils in an old 17th century treatise … What does it mean?
Folgen etliche Exempla diminutionum, sind aber nichtentheils auff Instrumenten […] zugebrauchen.
While not completely implausible as an obsolete German word, it's still quite implausible, and I am sure it's not what nichtentheils is. If this word had ever existed, it would appear in modern editions with the modernised spelling nichtenteils. But this spelling is completely unknown to Google.
I looked at the few Google Books hits for nichtentheils. Every single instance is a blatantly obvious OCR error. The actual word is mehrentheils, or in modernised spelling mehrenteils. In fraktur type, m looks similar to ni, e looks similar to c and r looks similar to t. Apparently a rogue spot over the right half of the initial m is enough to put Google's OCR algorithm on the wrong track. Still, presumably this can only happen because mehrenteils is a relatively rare word and (again presumably) not built into Google's OCR dictionary.
Mehrenteils is still (very rarely) used in formal language, but its synonyms größtenteils and meistens are vastly more common. The word translates as mostly or (quite literally) as for the most part.
Obviously it is a very bad idea to rely on OCR for reading old books. You should always use the original text. It really doesn't take long to get used to fraktur.
PS: I just realised that dirkt has made the same observation before in a comment, though he stated it less confidently.
Supposing the word "nichtentheils" ever existed: It's possible that "nichtentheils" has later been modified to "nichtenteils" as many Names and Words which now are written only with a "t" have been eralier written with "th" like: Thal=Tal.
This contains at least two words we actually use: "nichts" nothing and "Teil" Piece/Part or "nichtens" which is more or less equivalent to the expression "mit Nichten": Which is a kind of negation like "not even". Which seems to fit with the context of your sentence!
Therefore i would assume that "nichtentheils" means something like "not even partly" or "no part of it" or something like this.
Ein zusammengesetztes Wort aus:
Heutzutage würde man keinesfalls bzw. gar nicht bzw. überhaupt nicht verwenden. Siehe auch das im Amtsdeutsch gebräuchliche mitnichten (3).