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.

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 unfair or mean behavior. 'fies' is more general and means that someone is mean, malicious, or unfair to others. This can also be a small but unpleasant action. 'link' emphasizes more strongly that someone acts in a sneaky or dishonest way, meaning they do not play fair openly. Both words are rather colloquial. Learners may confuse them because both speak negatively about behavior. If it is mainly mean, 'fies' fits. If deception or a hidden trick is involved, 'link' fits better.

Die Bemerkung der Mitarbeiterin war fies und hat Tom verletzt.
The employee's remark was mean and hurt Tom.
Es war link von ihr, die Schuld heimlich auf ihre Kollegin zu schieben.
It was sneaky of her to secretly shift the blame onto her colleague.