This is the 645th most frequent Polish word.
siedmiu
The word "siedmiu" in Polish means "seven" (in the genitive or masculine personal plural form).
Here, 'siedmiu' is used in its genitive case form as it complements the plural noun 'cudach.'
Oni rozmawiali o siedmiu cudach świata.
They talked about the seven wonders of the world.
In this instance, 'siedmiu' is used as the masculine personal numeral referring to 'seven men.'
Zobaczyłem siedmiu mężczyzn w parku.
I saw seven men in the park.
In this sentence, 'siedmiu' is the masculine personal ordinal number referring to 'seven male persons.'