durcheinander
chaotic
In a state of disorder or chaos.
Nach dem Umzug waren alle Kisten und Möbel noch durcheinander.
After the move, all the boxes and furniture were still chaotic.

mixed up / chaotic
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chaotic
In a state of disorder or chaos.
Nach dem Umzug waren alle Kisten und Möbel noch durcheinander.
After the move, all the boxes and furniture were still chaotic.

The word is made up of 'durch' (through) and 'einander' (one another). The literal meaning 'through one another' creates a clear image of chaos and disorder.
Alle Fäden waren so durcheinander, dass man sie nicht mehr entwirren konnte.
All the threads were so tangled up that they could no longer be untangled.
'Durcheinander' (mixed up/chaotic) can be used as an adverb or as a predicative adjective. It can also be part of separable verbs such as 'durcheinanderbringen' (to confuse, to mix up) or 'durcheinanderkommen' (to get mixed up).
Bitte redet nicht alle durcheinander!
Please don't all talk over each other!
When 'durcheinander' (mixed up) refers to a person, it almost always means 'mentally confused' or 'emotionally upset.' When it refers to things or a place, it means 'messy' or 'disordered.'
Nach der langen Prüfung war er geistig völlig durcheinander.
After the long exam, he was mentally completely mixed up.
The separable verb 'jemanden durcheinanderbringen' (to confuse someone, to throw someone off) is a very common combination. It means to confuse someone or make them lose their train of thought.
Deine plötzliche Frage hat mich komplett durcheinandergebracht.
Your sudden question completely threw me off.
with each other
'miteinander' and 'durcheinander' can both occur when several things or actions come together. 'miteinander' describes more of a connection or mixture that can still be meaningful or intentional. 'durcheinander', on the other hand, emphasizes disorder, chaos, or an incorrect order. That is why the words can be confused when many things are together. If something is connected or mixed, 'miteinander' fits; if it is messy or confusing, 'durcheinander' fits.

Wegen der vielen Termine sind meine Notizen heute ganz durcheinander.
Because of all the appointments, my notes are completely mixed up today.
Für den Kuchen verrührt Lisa Mehl, Eier und Milch miteinander.
For the cake, Lisa mixes flour, eggs, and milk together.