This is the 962nd most frequent Polish word.
psa
The Polish word "psa" means "dog" (genitive singular form).
In this sentence, 'psa' is the accusative form of 'pies,' which is used because the dog is the direct object of the verb.
Mam psa.
I have a dog.
Here, 'psem' is the instrumental case form of 'pies,' used with the preposition 'z' to indicate accompaniment.
Idę z psem.
I am walking with the dog.
In this context, 'psa' is again in the accusative case of 'pies,' here governed by the negative past tense verb.