- Herze (non-standard nowadays)
From Middle High German herze, from Old High German herza, from Proto-Germanic *hertô (“heart”), from Proto-Indo-European *ḱḗr (“heart”). Cognate with Dutch hart, English heart, Danish hjerte, Gothic 𐌷𐌰𐌹𐍂𐍄𐍉 (hairto).
Herz has irregular singular declension and is the only noun of its kind.
- The genitive singular generally takes the ending -ens: des Herzens.
- The dative singular traditionally takes -en: dem Herzen. This form is still the only accepted standard form in many—more or less fixed—expressions, such as im Herzen, von Herzen, zu Herzen, Operation am offenen Herzen (“open-heart surgery”), mit halbem Herzen (“half-heartedly”), and more.
- Otherwise, the forms dem Herzen and dem Herz are both acceptable. The latter is predominant in speech, while the former remains the more established form in writing. — But only the bare form is common for Herz as a card suit or a term of endearment, as well as in the phrase mit Herz (“good-hearted”).
- Herz in Duden online