This is the 15727th most frequent Polish word.
szczęściem
"By fortune" or "luckily."
In this sentence, 'szczęściem' functions as an adverb-like expression implying 'fortunately.'
Szczęściem udało mi się zdążyć na autobus.
Fortunately, I managed to catch the bus.
Here, 'szczęściem' is used as a noun in instrumental case, indicating what the subject identifies as their greatest happiness.
Moim największym szczęściem jest rodzina.
My greatest happiness is my family.
In this example, 'szczęściem' again functions as an adverbial complement, conveying the element of luck in the narrative.