This is the 112th most frequent Czech word.
nejen
"Not only"
The word 'nejen' introduces the first part of a paired construction where something exceptional is mentioned (
Nejen že uměl hrát na klavír, ale také skvěle kreslil.
Not only could he play the piano, but he could also draw brilliantly.
Here, 'nejen' is used to couple two positive attributes of an item, emphasizing its qualities.
Tento stroj je nejen rychlý, ale i efektivní.
This machine is not only fast, but also efficient.
In this sentence, 'nejen' emphasizes that an unexpected or additional person was involved in the action.