
fies
mean
Mean, malicious, or unfair in behavior toward other people.
Es war wirklich fies von ihm, dich vor allen Leuten auszulachen.
It was really mean of him to laugh at you in front of everyone.
mean / nasty
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mean
Mean, malicious, or unfair in behavior toward other people.
Es war wirklich fies von ihm, dich vor allen Leuten auszulachen.
It was really mean of him to laugh at you in front of everyone.

nasty
Very unpleasant to the senses; disgusting or repulsive in taste, smell, or appearance.
Der Geruch aus dem Abfluss ist einfach nur fies.
The smell from the drain is just plain nasty.

bad / nasty
Colloquial for very strong, intense, or bad, usually in a negative context such as pain, cold, or a cold/illness.
Ich habe heute fiese Kopfschmerzen und kann mich kaum konzentrieren.
I have a nasty headache today and can hardly concentrate.
The word 'fies' (mean/nasty) is used almost exclusively in colloquial language. In formal or written contexts, it is rather uncommon.
Sag so was nicht, das ist fies!
Don't say something like that, that's mean!
The meaning of 'fies' (mean/nasty) depends heavily on the context. If it refers to a person or action, it means 'gemein' (mean). If it refers to smell or taste, it means 'ekelhaft' (disgusting). In connection with pain or weather, it means 'stark' (strong) or 'schlimm' (bad).
Sein Kommentar war wirklich fies. (gemein)
His comment was really mean. (mean)
When talking about mean behavior, you often hear combinations such as 'eine fiese Bemerkung' (a mean remark), 'ein fieser Trick' (a nasty trick), or 'ein fieses Grinsen' (a nasty grin).
Mit einem fiesen Trick hat er das Spiel gewonnen.
He won the game with a nasty trick.
Colloquially, 'fies' (bad/nasty) is often used as an intensifier for negative things. Typical examples are 'fiese Schmerzen' (bad pain), 'eine fiese Kälte' (a biting cold), or 'ein fieser Muskelkater' (bad muscle soreness).
Draußen herrscht eine fiese Kälte.
There is a biting cold outside.
sneaky / left
Link and fies can both describe an unpleasant or unfair action. Link particularly emphasizes deceit, dishonesty, or a fraudulent trick; fies refers more generally to cruel or mean behavior. Choose link when the deception is important, and fies when the main point is the maliciousness.

Der fiese Trick machte sich über seine Unwissenheit lustig.
The nasty trick mocked his ignorance.
Der linke Trick brachte ihn dazu, freiwillig mehr Geld zu bezahlen.
The sneaky trick persuaded him to pay more money voluntarily.