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The plural of Tumor is not, as one might have expected, Tumore, but Tumoren.

The Duden says

der Tumor; Genitiv: des Tumors, Plural: die Tumoren, umgangssprachlich auch: Tumore

So Tumore is just the colloquial plural and sounds more natural. I hear many German native speakers say Tumore, even physicians. That's why I'm curious about the origin of Tumoren.

What's the etymology of Tumoren as the plural form of Tumor?

_{I'm aware of this earlier question in German about Skript and Tumor, which is also tagged etymology but doesn't seem to ask about the history of that plural and the only answer doesn't touch upon it either, even saying they never heard Tumoren. They seem to be from Austria, so maybe there are regional differences. So it doesn't answer my question.}

Actually, the question is what's the origin of the colloquial form "Tumore". "Tumoren" is formed quite regularly: "Motoren", "Rotoren", "Professoren", "Laudatoren", etc.

Fremdwörter scheinen den n-Plural zu mögen. Man denke an Fälle mit ersetzendem n-Plural:

Them-a, Them-en; Kont-o, Kont-en; Vir-us, Vir-en

Viele Fremdwörter auf -or, bei denen die Pänultima – die Sibe vor dem -or – betont ist, haben n-Plural. Im Plural wechselt die Betonung und -or- wird betont.

'Autor, Di'rektor, Pro'fessor; 'Faktor, Gene'rator, 'Sensor

Bei Wörtern, die im Singular auf betontes -or enden, findet man dagegen Beispiele mit e-Plural.

Kon'tor, Tre'sor (auch bei Betonung der Antepänultima: 'Monitor)

Bei Tumor bezeichnet der Duden die Betonung auf der zweiten Silbe als umgangssprachlich. Betonung auf der zweiten Silbe paßt besser zum e-Plural als zum n-Plural; das könnte die Entstehung der Pluralvarianten erklären.

Man vergleiche auch 'Motor, Mo'tor, wo neben dem üblichen n-Plural ebenfalls ein e-Plural existiert.

As you have already mentioned, both plural forms exist. The rule, when to have -e and when to have -en in the nom. pl. of words ending in -or depends on which syllable of the singular is stressed.

If the last syllable before the ending -or ist stressed, the nom. pl. ends in -en (' before the syllable marks the stress):

• 
  der 'Autor -> die Aut'oren
  


• 
  der Di'rektor -> die Direkt'oren
  


• 
  der Akkumu'lator -> die Akkumla'toren
  

As you can see, the stress also moves one syllable to the right in the nom. pl.

If the ending -or ist stressed, the nom. pl. ends in -e (' before the syllable marks the stress):

• 
  der Ma'jor -> die Ma'jore
  


• 
  der Te'nor (singer) -> die Te'nöre (here also with umlaut as an extra marker for the plural)

Finally, there are -or-words where the stressed syllable varies within the German pronunciation and because of that the ending of the nom. pl. varies accordingly. E. g. (' before the syllable marks the tress):

• 
  der 'Motor/der Mo'tor -> die Mo'toren/die Mo'tore
  


• 
  der 'Tumor/der Tu'mor -> die Tu'moren/die Tu'more
  

If you have a look at the Duden-entry on Tumor, you can see, that the colloquial accentuation indeed can also be Tu'mor.