This is the 13935th most frequent Polish word.
portfelu
"Portfelu" is the locative or genitive form of "portfel," meaning "wallet" in English.
Here, 'portfelu' is the locative case of the noun 'portfel', used because it follows the preposition 'w' (in).
Mam dużo pieniędzy w portfelu.
I have a lot of money in my wallet.
It again uses the locative case due to the preposition 'w', now with an adjective describing the color of the wallet.
Nie mogę znaleźć niczego w moim czarnym portfelu.
I can't find anything in my black wallet.
'Portfelu' remains in the locative case, the structure emphasizing the contents of the wallet.