Irene Schlotter, Dipl.-Übers. Aug 9, 2022:

Definition lt.Merriam Webster/Cambridge Dictionary Merriam Webster:
Definition of put up or shut up
informal
—used to tell someone in a somewhat rude way to ***start doing something or to stop talking about it*** You've complained long enough. It's time to put up or shut up.

Cambridge Dictionary:
put up or shut up
idiom informal
If you say someone should put up or shut up, you mean that they should ***either take action in order to do what they have been talking about, or stop talking about it***:
You keep saying you're going to ask her out. Well, put up or shut up.

Janet Muehlbacher (asker) Aug 7, 2022:

it´s about a family member talking about another relative behind their back, instead of saying it to their face

Rolf Keller Aug 7, 2022:

Kontext??????????????????????????????? "Tu es oder halt den Mund."
Aber es gibt natürlich noch 999 andere Möglichkeiten.