The German adjective fertig is typically translated as "finished" or "done," and we generally see it used on Yabla in its most common standard usage:
So, mein Apfelkuchen ist jetzt fertig.
So, my apple cake is now done.
Caption 37, Apfelkuchen - mit EvaPlay Caption
There are, however, a number of slang uses of the adjective, including its inclusion in the separable verb fertigmachen:
Ich mache euch fertig!
I'll finish you off [idiom: retaliate]!
Caption 12, Die Pfefferkörner - EigentorPlay Caption
According to the Duden German dictionary, the definitions of the slang word word fertigmachen (or fertig machen) are: 1. to issue a sharp rebuke or reprimand; 2. to break or wear out somebody's will, to drive them to desperation; 3. to completely vanquish or physically destroy or kill; 4. to satisfy sexually or bring to an orgasm. The above translation is closest to the second and third meanings.
Probably the most common slang usage that you'll hear in everyday spoken German means "tired":
Wir sind fix und fertig, hä?
We are worn out, huh?
Caption 4, Deutschkurs in Blaubeuren - Der RelativsatzPlay Caption
The above video uses the full phrase fix und fertig, but even using only the word fertig in the right context is enough:
Und dann war ich wieder völlig fertig...
And then I was completely down again...
As you see here, fertig is translated as "down" in the sense of tired, or worn out, or in the American vernacular: blue!
Do a search on German Yabla for the word fertig to find some examples of its usage in a real-world context.