This is the 12639th most frequent Polish word.
nerkach
"Nerkach" means "kidneys" in the locative case in Polish.
The word 'nerkach' in this context refers to a beltbag or fanny pack.
Mam dokumenty w nerkach.
I have documents in the kidney-shaped pouch.
Here, 'nerkach' refers to kidneys in the physiological sense.
Czułem ból w nerkach po całym dniu.
I felt pain in my kidneys after the whole day.
The term here can refer to the kidney region in a more general or metaphorical anatomical sense.