This is the 18035th most frequent Russian word.
форменную
"Formенную" means "formal" or "uniform" depending on context.
The word 'форменную' is used as an adjective here to describe the type of clothing, specifically uniform clothing required for her job.
Она носила форменную одежду на работе.
She wore uniform clothing at work.
The word 'форменную' modifies 'кепку,' indicating that the cap resembles the style used in official or military uniforms.
Он купил себе форменную кепку, как у военных.
He bought himself a uniform cap like the ones used by the military.
Here, 'форменные' describes 'наряды,' referring to the uniforms that restaurant staff are required to wear.