This is the 12110th most frequent Polish word.
lustrem
The Polish word "lustrem" means "mirror" (instrumental case).
Here, 'lustrem' is used as the instrumental case of the noun 'lustro', meaning mirror. It specifies the instrument or means through/by which the action is performed or the manner in which it occurs.
Patrzyłem na swoje odbicie w lustrem.
I looked at my reflection in the mirror.
In this sentence, 'lustrem' again appears in instrumental case, indicating the means used to achieve a particular result or to describe how something is done.
Stół został przykryty lustrem, aby wyglądał bardziej elegancko.
The table was covered with a mirror to make it look more elegant.
Here as well, 'lustrem' is in instrumental case, showing the means or tool used by the subject to carry out an activity.