die Privatsache
private matter
A personal matter that concerns only the person involved and is not intended for the public.
Ob ich heirate oder nicht, ist meine Privatsache.
Whether I get married or not is my private matter.

private matter
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private matter
A personal matter that concerns only the person involved and is not intended for the public.
Ob ich heirate oder nicht, ist meine Privatsache.
Whether I get married or not is my private matter.

The word is made up of the adjective 'privat' (private) and the noun 'die Sache' (the matter/thing). So it literally means a matter that is not public, but personal.
Seine Finanzen sind seine Privatsache.
His finances are his private matter.
The word is very often used with a possessive pronoun to set a personal boundary. The statement 'Das ist meine Privatsache' (That is my private matter) means that you do not want to talk about a topic.
Was ich in meiner Freizeit mache, ist meine Privatsache.
What I do in my free time is my private matter.
The phrase 'etwas ist reine Privatsache' (something is purely a private matter) is often used to emphasize that a matter is absolutely no one else's business. The adjective 'rein' (pure) strengthens the statement here.
Die Entscheidung, umzuziehen, ist reine Privatsache der Familie.
The decision to move is purely the family's private matter.
secret
'das Geheimnis' and 'die Privatsache' both concern things that are not meant for everyone. A 'Geheimnis' is specific information that should remain hidden or that only a few people know. A 'Privatsache' is a personal matter that is nobody else's business. It does not necessarily have to be secret, but you want to decide for yourself whether to talk about it. Learners may confuse the words because both have to do with personal information. 'Geheimnis' emphasizes hiding a piece of information, while 'Privatsache' emphasizes a personal boundary.

Seine Krankheit ist eine Privatsache, deshalb fragt niemand weiter nach.
His illness is a private matter, so no one asks any further questions.
Im Bus hat Paul aus Versehen ein Geheimnis über die Reise verraten.
On the bus, Paul accidentally revealed a secret about the trip.