Rashi on Exodus 13:2/רש"י על שמות י״ג:ב׳

1פטר כל רחם THE FIRST OFFSPRING OF EVERY WOMB — The words mean: that which first openeth the womb, the root פטר meaning to open, as in (Proverbs 17:14) “As one makes an opening (פוטר) for water so is the beginning of strife”; and so, too, is (Psalms 22:8) יפטירו בשפה i. e. “they open the lips”.

2לי הוא IT IS MINE — I have acquired him for Myself through My having smitten the first born of the Egyptians.