
fix
quick
(colloquial) Quick; in a short time.
Ich muss nur noch fix die E-Mails checken, dann können wir gehen.
I just need to quickly check my emails, then we can go.
quick / fixed
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quick
(colloquial) Quick; in a short time.
Ich muss nur noch fix die E-Mails checken, dann können wir gehen.
I just need to quickly check my emails, then we can go.

fixed
Set, unchangeable, or permanently installed.
Die Miete ist ein fixer Kostenpunkt in unserem monatlichen Budget.
Rent is a fixed cost item in our monthly budget.

to be exhausted
(idiom) To be completely exhausted, worn out, or also mentally at the end of one's rope.
Nach der anstrengenden Arbeitswoche war ich am Freitagabend einfach nur fix und fertig.
After the exhausting workweek, I was simply completely wiped out on Friday evening.
The German adjective 'fix' means 'quick' or 'fixed'. It is a false friend of the English verb 'to fix', which in German means 'reparieren' (to repair).
Kannst du bitte mein Fahrrad reparieren?
Could you please repair my bicycle?
In the meaning 'schnell' (quick), 'fix' is mostly used colloquially and as an adverb. It then remains unchanged in the sentence and describes an action.
Warte kurz, ich hole nur fix meine Jacke.
Wait a moment, I'm just quickly getting my jacket.
In the meaning 'feststehend' (fixed), 'fix' is often used in economic or organizational contexts. Typical combinations are 'fixe Kosten' (fixed costs), 'ein fixer Termin' (a fixed appointment), or 'ein fixer Preis' (a fixed price).
Wir haben einen fixen Termin für das nächste Treffen vereinbart.
We agreed on a fixed date for the next meeting.
The expression 'fix und fertig sein' means to be completely exhausted. The two words 'fix' and 'fertig' are always used together here.
Nach dem Umzug war ich tagelang fix und fertig.
After the move, I was completely wiped out for days.
fast / quick
Schnell and fix can both mean that an action is completed in a short time. Schnell is neutral and works in both everyday and more formal situations. Fix is distinctly more colloquial and is especially suitable for casual conversations.

Mein Freund sagt: Schick mir die Datei bitte fix.
My friend says, “Please send me the file real quick.”
Die Chefin sagt: Schicken Sie mir die Datei bitte schnell.
The manager says, “Please send me the file quickly.”