der Radfahrer
cyclist
A person who rides a bicycle.
Auf dem neuen Radweg sind viele Radfahrer unterwegs.
Many cyclists are out on the new bike path.

cyclist
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cyclist
A person who rides a bicycle.
Auf dem neuen Radweg sind viele Radfahrer unterwegs.
Many cyclists are out on the new bike path.

The word is made up of 'das Rad' (here in the sense of bicycle) and 'der Fahrer' (driver/rider). So it literally describes a person who rides a bike.
Achtung, hier überqueren oft Radfahrer die Straße.
Careful, cyclists often cross the road here.
The figurative meaning is based on the body position when cycling: you hunch upward ('buckelt nach oben' — submissive toward superiors) and kick downward ('tritt nach unten' — condescending toward subordinates). This image helps you remember the meaning.
Sein Verhalten ist typisch für einen Radfahrer: zu den Chefs nett, zu uns unmöglich.
His behavior is typical of a toadying bully: nice to the bosses, impossible to us.
In the figurative meaning, 'Radfahrer' (toadying bully) is always a derogatory description of a character trait. It is a criticism or an insult, not a neutral statement.
Niemand im Team mag ihn, weil er so ein Radfahrer ist.
Nobody on the team likes him because he is such a toadying bully.
driver
'der Fahrer' and 'der Radfahrer' both refer to people who use or operate a vehicle. However, 'der Fahrer' is much more general: it can be a person in a car, bus, truck, train, on a motorcycle, or even on a bicycle. 'der Radfahrer' specifically means a person who rides a bicycle. The words can be confused because a cyclist is also a type of driver or rider. If the bicycle is important, say 'Radfahrer'. If the vehicle is not named exactly, or if it is about a car, bus, or similar vehicle, 'Fahrer' fits better.

Nach der Nachricht fuhr Anton als Radfahrer besonders vorsichtig durch die Stadt.
After the news, Anton rode through the city especially carefully as a cyclist.
Der Fahrer des Busses wartet, bis alle eingestiegen sind.
The bus driver waits until everyone has boarded.