Why is it die Diffamierung?

Diffamierung ends in -ung, and nouns ending in -ung are feminine: in a German dictionary, 99% of them take die.

All nouns ending in -ung
Sense 1
die

Meaning

[1] The targeted defamation of third parties

[1] die gezielte Verleumdung Dritter

Examples

[1] The defamation of political opponents is one of the common forms of the power struggle

[1] Die Diffamierung politischer Gegner gehört zu den gängigen Formen des Machtkampfes.

Declension

1 Singular Plural
Nominativ die Diffamierung die Diffamierungen
Genitiv der Diffamierung der Diffamierungen
Dativ der Diffamierung den Diffamierungen
Akkusativ die Diffamierung die Diffamierungen

Pronunciation

Dictionary entries include material from Wiktionary. Wiktionary